Allan Gaw wo pro Deputy Study Director Robert A. Cowan ssc rno Lecturer in Pathological Biochemistry Denis St. J. O'Reilly tsemo rrcratn Consultant Clinical Biochemist Michael J. Stewart pho raceatn Consultant Scientist James Shepherd pho rrce rrceath ase Professor of Pathological Biochemistry Department of Pathological Biochemistry University of Glasgow and Glasgow Royal Infirmary University NHS Trust Glasgow, UK Illustrated by Robert Britton D crurcrn Srstohe EDNEURGH LONDON NEW YORK PHILADELPHIA SYONEY TORYO 1908CONTENTS ‘The clinical biochemistry laboratory ‘The use of the laboratory 4 ‘The interpretation of results 6 Biochemical testing outside the laboratory 8 Reference ranges 10 CORE BIOCHEMISTRY ) 1 Fluid and electrolyte balance: concepts and vocabulary 12 Proteins and enzymes 44 Water and sodium balance 14 Immunoglobulins 46 Hypermatraemia 16 Myocardial infarction 48 Hyponatraemia (1) 18 Liver function tests 50 Hyponatraemia (2) 20 Jaundice $2 Potassium disorders, Liver disease 54 Intravenous fluid therapy 24 lucose metabolism and diabetes mellitus 56 Investigation of renal function (1) 26 Diagnosis and monitoring of diabetes mellitus 58 Investigation of renal function (2) 28 Diabetic ketoacidosis 60) Acute renal failure 30 Hypoglycaemia 62 Chronie renal failure 32 Calcium regulation and hypocalcaemia 64 AAcid-base: concepts and vocabulary 34 Hypercaleaemia 66 Metabolic acid-ase disorders 36 Phosphate and magnesium 68 Respiratory and mixed acid-base disorders 38 Bone disease 70 Oxygen tranyport 40 Osteoporosis 72 Acid-base disorders: di osis and management 42 ENDOCRINOLOGY n Endocrine control 74 Adrenocortical pathophysiology 86 Pituitary function 76 Hypofunetion of the adrenal cortex 88 Growth disorders and acromegaly 78 Hyperfunetion of the adrenal cortex 90 ‘Thytoid pathophysiology 80 Gonadal 2 Hypothyroidism 82 Subfertility 94 Hyperthyroidism 84 Nutritional assessment Aleohol 116 ‘Nutritional support 98 Coma 118 Parenteral nutrition 100 Lipoprotein metabolism 120 ‘The metabolic response to injury 102 Clinical disorders of lipid metabolism 12, Malabsorption 104 Management of hyperlipidaemia 124 Iron 106 Hypertension 126 Copper and zine 108 Cancer and its consequences 128 ‘Therapeutic drug monitoring 110 Tumour markers 130 Toxicology 112 Gut hormones and multiple endocrine neoplasia 132 Metal poisoning 114 Hyperuricaemia 134vt ‘CONTENTS Skeletal muscle disorders 136 DNA diagnosis 138 Fetal monitoring and prenatal diagnosis 140 Pregnancy 142 Screening the newborn for disease 144 Case history comments 152 Index 160 Inherited disorders 146 Paediatric biochemistry 148 Biochemistry in the elderly 150 tClinical biochemistry, chemical pathology and clinical chemistry are all names for the subject of this book, that branch of laboratory medicine in which chemical and biochemical methods are applied 10 the study of disease (Fig. 1), While in theory this embraces all non-morphological studies, practice tis usually, though not cly, confined to studies on blood because of the relative ease in obtaining such specimens although analyses are made on other body fluids such as gastric aspirate and cerebrospinal fluid, Clinical biochemical tests comprise ‘over one-third of all hospital laboratory investigations. ‘THE USE OF BIOCHEMICAL TESTS Biochemical investigations are involved, to varying degrees, in every branch of clinical medicine. The results of bio- chemical tests may be of use_in diagn sis and in the monitoring of treatment Biochemical tests may also be of value in screening for disease or in assessing t prognosis onceadiagnosishasbeen made (Fig. 2). The biochemistry laboratory is, often involved in research into the biochemical basis of disease and in clinical trials of new drugs. “tines” owe ocharaty Fi, 1 The place of trical bochemistr in medi CORE BIOCHEMISTRY iemical facilities are providedinevery bough not necessarily to the ‘same extent, Most biochemistry laborato- Fes provide the ‘core analyses’, commonly requested tests which are of value in many patients, ona frequent basis (Table 1). The clinician will often request specific groupingsofests, and clinical biochem- istry assumes a cryptic language of its ‘own as request forms arrive at laboratory reception for "U and Es’ (urea and electro Iytes), LFTs diver function tests) or“bloodt gases SPECIALIZED TESTS ‘There are a variety of specialties within clinical biochemistry (Table 1). Not every laboratory is equipped to carry out all possible biochemistry requests, Large departments may act as ref where less commonly asked for tests are performed, Forsome testswhicharenceded inthe diagnosis of rare diseases, there may be just one or two laboratories in the country offering the service, 1 cenies ‘Table 1 The clinical biochemistry repertoire ‘THE EMERGENCY LAB All clinical biochemistry laboratories provide facilities for urgent tests. Only a sinall number of test types are available from the “emergency laboratory’. These are processed rapidly and reports phoned Fig 2 How biochemical tests are used.Fa.9 Analysing the samples: a) he stomata to the requesting clinician or ward, An urgent testis desi the clinician is likely to take immediate xction. An ‘oncall’ service may be provided ated as one on which outside the normal working hours of the laboratory. Table 1 shows some of the IN THE OPERATING SUITE OR CLINIC In some large hospitals. the facilities to perform biochemistry analyses are sited loseto where they are needed, forexample :monitoring of patients undergoing major ¥, such as transplantation, oF pro- blood glucose esultsat the diabetic i clinic, Many biochemical tests are now performed away from the lubora tory (see pp. 8-9. AUTOMATION AND COMPUTERIZATION Most laboratories are now computerized and the use of bat-coding of specimens and automated methods of analy sis allows ahigh degree of productivity and improves the quality of service. Links to computer terminal mn wards and GP surgeries allow direet access to results by the requesting TEST REPERTOIRE There ane over 400 different tests which may be labo sartied out in clinical biochemistry jories. They vary from the very simple, such as the measurement of sodium, tothe highly complex, such as DNA analysis, lipoprotein variants. Many high volume ‘THE CLINICAL BIOCHEMISTRY LABORATORY nalyser (“ki analyses ant () mana methods, testsaredoneon large automated machines Less frequently performed tests may be conveniently cartied out by usin nts pa in ‘kit out manu: cially prepared rea form, Some analyses ally (Fig. 3). Increasing workload, and bud Niabo. most costeflective way of providing the best service Dynamic wsts require several s mens, timed in relation to a biochemical stimulus, such as a glucose load in the slucose tolerance test for the diagnosis of diabetes mellitus. Some tests provide clearcut answer to a question; others are only a part ofthe diagnostic jigsaw. This book describes how the results of biochemistry analyses are interpreted, rather than how the nalysesare performed inthe laboratory, An important function of many departments is research and Clinical note development. Advances in analytical methodology and in our understanding of disease continues to change the test reper toire ofthe biochemistry departments the value of new tests is appreciated LABORATORY PERSONNEL As well as performing clinical biochemistry laboratory also pro vide: tory has on its staff both medical and scientific personnel who are familiar with the clinical significance and the analytical performance of the te dures, and they will readily giv ne analyses, the «consultative service, The labora proce: the interpretation ofthe results. Do not be hesitant to take advantage oF this advice. especially where a case is not steuightfor ward. The clinical biochemistry laboratory © Biocmical tess are used in agnosis, rontrg eaten, rein and ir pogoss. ‘Care biochemical tess are cared uti ever biochemist laboratory. Speciaized esis may be refered o larger deparmens. Al hospitals provide for urgent issn he “emergency laboratory * Laboratory personel lea oe ate, based on tet knowledge and expeence, on te use of he Deceit labora, on the appropiate slctn oft, and about te interpretation of resus.Every biochemistry analysis should attempt to answer a question whieh the clinician has posed about the patient Obtaining the correet answers ean often to be fraught with difficulty SPECIMEN COLLECTION Inorder to carry out biochemical analyses, in is necessary that the laboratory be provided with both the correct specimen Forthe requested est,and alsoinformation which will ensure that the right test is ‘curried out and the result retumed to the requesting clinician with the minimum of delay. As much detail as possible should be included on the request form to help both laboratory staff and the elinician in the interpretation of result. This informa tion ean be very valuable when assessing ‘patient's progress over a period, or reas- sessing a diagnosis, Patient identification ‘mustbe correct. andthe requestformshould include some indication of the suspected pathology. The requested analyses should he clearly indicated. Request forms differ in design, Clinical biochemistry forms in Europe are conventionally coloured green, ‘A variety of specimens are used in biochemical analysis and these are shown in Table 1. However, the majority of bio- chemical tests are performed from venous blood or urine. Blood specimens blood is collected into a plain tube and allowed (© elot, after centrifugation a serum specimen is obtained (Fig. 1). For many biochemical analyses this will be the specimen recommended. In other ceases, especially when the analyte in question is unstable and speed is Pisin wena “smteosgela (ict tors | 1 + Whole Boo anaiyss Red eal | ‘aly + Lipide an Tpoprtens Ta antesaguant contra SST cI General | + General Fa.1 Blood specimen tubes fr specific biochemical tests. Te coloured necessary to obtain a specimen which can be frozen quickly, the blood is collected into a tube containing an anticoa, lant such as heparin. When centrifuged, the supernatant is called plasma which is almost identical to the cell-free fraction of blood but contains the anticoagulant as well. Urine specimens Urine specimen containers may include a preservative 10 inhibit bactcrial growth, or acid to stabilize certain metabolites. They need to be large enough to hold a full 24 h collection, Random urine samples are collected into small ‘universal” containers, Other specimen types For some tests, specific body fluids or tissue may be required. There will be specific protocols for the handling and transport of these samples to the labora- tory. Consult the local lab for adviee Table 1 Specimens used for biochemical analyses Vers bot serum asm, ari ot Capa od hun | Rage heparn | ‘Sxlate annsagulant sGonerai | + Glueose tacate ‘yautainrs in use n the authors hospital and laboratory Dangerous specimens All specimens from patients with dan- _gerous infections should be labelled with low “dangerous specimen’ sticker. A Similar label should be attached 10 the request form. Of most concem to the laboratory staf ate hepatitis B and HIV, but alf specimens should always. be treated both by clinicians and biochemis- try staf ay potentially hazardous. ‘SAMPLING ERRORS ‘There are a number of potential errors which may contribute (0 the success or failure of the laboratory to provide the correct answerstotheclinician’squestions ‘Some of these problems arise when a elinician first obtains specimens from the patient © Blood sampling techuique. Difficulty in obtaining ablood specimen may lead 10 hhaemolysis with consequent release of potassium and other red cell constitu. ents, Results for these will be falsely elevated. © Prolonged stasis during venepuncsure, Plasma water diffuses into the inte stitial space and the serum oF plasma ample obtained will be concentrated. Proteins nd protein-bound components of plasma such as calcium or thyroxine will be falsely elevated, © Insufficient specimen. Each biochemi- cal analysis requires acertain volume of specimen to enable the tes to be carried. out. It may prove to be impossible for the laboratory to measure everything requested on a small volume specimen, © Errors in timing. The biggest source of error in the measurement of any analyte in a 24-hour urine specimen is in the collection of an accurately timed vol- tume of urine. Incorreet specimen container. Formany analyses the blood must be collected. into a container with anticoagulant and preservative. For example, samples for ilucose should be collected into a spe- cial container containing fluoride which inhibits glycolysis: otherwise the time taken to deliver the sample to the labo- ratory can affect the result. Ira sample is collected into the wrong container, itshould neverbe decanted into another type of tube. For example, blood which hasbeen exposedeven briefly to EDTA {an anticoagulant used in simple con- tainers for lipids) will have a markedly reduced calcium concentration, ap- proaching zero,© Inappropriate sampling site. Blood samples should not be taken “down- Stream’ from an intravenous drip, It is ‘not unheard of for the laboratory to receive a blood glucose request on a specimen taken from the same arm into which 5% glucose is being infused. Usually the results are biochemically incredible but itis just possible that they may be acted upon, with disastrous consequences for the patient. © Incorrect specimen storage. A blood sample stored overnight before being sent 9 the laboratory will show falsely Cini high potassium, phosphate and red cell enzymessuchas lactate dehydrogenase, because of leakage into the extracellular ‘uid from the cells, HOW OFTEN TO INVESTIGATE Many biochemical tests are repeated at intervals, How often depends on how quickly significant changes are liable to ‘occur,and there is little point in requesting tests ifanumerical change will ot havean influence on treatment Fa,2 Cheuit diagram ofthe clinical biochemistry process. Case history 1 blood specimen was aken from a 65-year-old woman to check her serim potassium concentration as ‘she had been on thiaride diuretics for some time, The GP left the specimen in his car and dropped itoff atthe laboratory on the way to the surgery the next moming. ‘Immediately on analysing the sample, the biochemist was on the ‘phone to the GP. Why? Comment on page 152. Clinical note URGENT REQUESTS ‘The main reason for asking for an analy sis to be performed on an urgent basis is that immediate treatment depends on the result ANALYSING THE SPECIMEN ‘Once the form and specimen arrive atthe laboratory reception, they are matched ‘with a unique identifying number oF bar code, The average lab receives: many thousands of requests and samples each day and its important that all are clearly identified and never mixed up. Samples proceed through the laboratory as shown in Figure 2. All analytical procedures are ‘quality controlledandithe laboratory strives for reliability ‘Once the results are available they are collated and areportisissued. Cumulative reports allow the clinician to see at a sslance how the most recent result(s) compare with those tests performed previously, providingan aid tothe moni- toring of treatment UNNECESSARY TESTING There can be no definite rules about the appropriateness, or otherwise, of labora tory testing because ofthe huge variety of clinical circumstances which may ar Clinicians should always bear in mind that in requesting a biochemical test should be asking a question ofthe labor tory. IF not, both the clinician and the laboratory may be performing unneces- sary work, with little benefit to the patient.‘THE LABORATORY REPORT ean take considerable effort, and expense, to produce what may seem to be just numbers on pieces of paper. Understand ing What these numbers mean is of erucial ‘importance ifthe correct diagnosis toe made, or ifthe patient's treatment is to be changed, HOW BIOCHEMICAL RESULTS ARE EXPRESSED Most biochemical analyses are quantita live, although simple qui quantitative tests sueh as those for the presence of glucose in urine are commonly encountered in methods. used for bio- chemistry testing away from the labora- tory. Many (ests measure the amount of the analyte in a small volume of the sam: ple, whether that is blood, plasma, serum, rine oF some other fluid or tissue, The test results are commonly expressed in ‘molar units. A mole of any compound always contains 6 x 10° molecules. Describing how much of an analyt present in ‘moles’ indicates how many ‘molecules of the substance are present Molar units ean be converted to, mass units; one mole isthe molecular weight of the substance in grams. Results are reported as concentrations, usually in terms ofthe number of moles in ‘one litre (mol) (Table 1). The concept of concentration is ilus- tated in Figure 1. The concentration of any analyte in a body compartment is ratio: the amount of the substance dissolved ina known volume. Changes in concen- {ration can oecur for two reasons: ‘© The amount of the analyte can increase or decrease. ‘© The volume of tTuid in which the analyte is dissolved ean similarly change. Enzymes are not usually expressed in ‘moles but as enzyme aetivity in “units” Enzyme assays are carried out in such a Table 1 Molar units way that the activity measured is directly proportional to the amount of enzyme present. Some hormone measurements are expressed as “units” by comparison to standard reference preparationsof known biological potency. Large molecules such as proteins are reported as grams or milli- _grams. Blood gas results (PCO, or PO.) are expressed in kilopascals (KPa), the ‘units in which partial pressures. are measured, VARIATION IN RESULTS Biochemical measurements vary for 1wo reasons, There is analytical variation, and also biological variation, Laboratory analytical performance A mumber of terms describe biochemical results, These include: ® precision and accuracy © sensitivity and speci © Fp. Understanding concentrations, ‘Concentration is always dependent on 0 factors: the mount of solute and he amount ‘of solvent, The concentration of the sugar Solution in the eer ean be increase from 1 Spoon/oeaker (@) to 2 spoomvtoaker by ether devreasng the volume of savent (B) Increasing the ant of sole (@) pr Fig. 2 Precision and accuracy. Precise but inaceurate © quality assurance *# reference ranges. Precision and accuracy Precision isthe reproducibility of an ana- lytical method, Accuracy defines how close the measured value isto the actual value. A good analogy is that of the shooting {urget. Figure 2shows the scatter of results ‘which mightbe obtained by someone with litle skill, compared with thatof someone ‘with good precision where the results are closely grouped together. Even when the results are all close, they may not hit the centre of the target. Accuracy is therefore poor, as if the “Sights” are off, It is the objective in every biochemical method to hhave good precision and accuracy Sensitivity and specificity Sensitivity ofan assay isa measure of how Title of the analyte the method can detect As new methods are developed the ‘offer improved detection limits which may help inthe discrimination between normal results and those in patients with the suspected disease. Specificity of an assay relatestohow good the assay'is at discrimi= nating between the requested 3 potentially interfering substances. Quality assurance Every laboratory takes great pains to eensute that the methods in use continue to produce refiale resulls. Laboratory staif monitor performance of assays using ‘quality control samples to give reassurance that the method is performing satistuc- totily with the patients” specimens. These are internal quality controls which are analysed every day or every time an assay isrun. The expected values are known and the actual results obtained are compared wilh previous values t monitor per- formance. In external quality assurance ‘hemes, identical samples are distributed {o laboratories; results are then compared. Precise and accurateIn this way, the laboratory's own internal sandards are themselves assessed. Reference ranges, ‘Analytical variation is generally less than that from biological variables. Biochemical test results are usually compared to a reference range considered to represent the normal healthy state (Fig. 3). Most reference ranges are chosen arbitrarily to include 95% of the values found it healthy volunteers, and hence, by defini tion, 5% of the population will have a result outwith the reference range. In practice there are no rigid limits demareat- ing the diseased population from the healthy; however, the further a result is from the limits of the range, the more likely itis to represent pathology, In some situstionsitisusefultodefine ‘action limits", ‘where appropriate intervention should be ‘made in response to. biochemical result ‘There is often a degree of overlap between the disease state und the ‘normal value’ (Fig. 4). A patient with an abnormal result who is found not tohave the disease is a false posiive. A patient who has the disease but has anormal” result isa false negative. Biological factors affecting the interpretation of results ‘The discrimination between normal and abnormal results is affected by various physiological factors which must be con- sidered when interpreting any given result. These include: *# Sex ofthe patient, Reference ranges for some analytes such as serum creatinine are different for men and ‘Age of the patient. There may be lifferent reference ranges for neonates, children, adults and the elderly © fect of diet. The sample may be inappropriate if taken when the patient is fasting or after a meal Time when sample was taken. There may be variations during the day and night. Stress and anxiety. These may alfeet the analyte of interest Posture of the patient, Redistribution of fluid may affect the result. © Effects of exercise. Strenuous exercise cam release enzymes from tissues, © Medical history. Infection and/or tissue injury can affect biochemical values independently of the disease process being investigated Pregnancy. This alters some reference ranges, Menstrual eycle. Hormone measure- ments will vary through the menstrual oycle. “180. Mean 18D +250 Measured van. "ean? 25D = Reference range Healy torneo range ————! Davased reference range Fi 4 Overiap of biochemical results in health and disease. © Drug history. Drugs may have specific effects on the plasma concentration of some analytes. Other factors ‘When the numbers have been printed on the report form, they still have to be inter- preted in the light of a host of variables. ‘Analytical and biological variations have already been considered. Other factors relatetothe patient. Thecliniciancan refer to the patient or w the clinical notes, ‘whereas the biochemisthesonly the infor- ‘mation on the request form to consult, The cumulation of biochemistry results is often helpful in patient management, Case history 2 ‘A serum potassium concentration of 45 mmol was recorded in the notes of a 35-year-old man being ‘prepared for appendicectomy. The ‘set of electrolyte results had been ‘phoned from the laboratory. The vas unperturbed although he did check the results on ‘the ward terminal himself, Why? Comment on page 152 ‘The clinician may well ask the follow- ing questions on receiving a biochemistry report © "Does the result fit im with what 1 expected on the basis ofthe clinical examination and history of the ‘© "IFthe result is not what I expected, can T explain the diserepaney?” ‘© “How can the result change my diagnosis or the way Tam managing the patient?” © What should I do next?” ‘What is done in response toa istry report rests with the elinical judge- ‘ment of the doctor. There isa maxim that doctors should always “1reat the patient, rather than the laboratory report” The rest of this book deals with the biochemical investigation of patients and the interpre- tation of the results obtained. Clinical note cis importam to realize that an abnormal resu does not always indicate that a isease is present, nor a normal result that it is not. Beware of over: reacting to the slighily abnormal result in the otherwise healthy vidual, woeINTRODUCING CLINICAL BIOCHEMISTRY BIOCHEMICAL TESTING OUTSIDE THE LABORATORY INTRODUCTION The methods for measuring some biologi cal compounds in blood and urine have use that ‘measurements ean be made away from the become so robust and simple laboratory—by the patient’s bedside, in the ward sideroom, at the GP's surgery in the home or even in the shopping centre! Convenience and the desite to Know results quickly ul profit by the manufac turers of the tests, have been the major 1s well as expect tion of commer stimuli for these developments, Experi- tence has shown that motivated individu als, e.. diabotics, frequently perform the tests ay well as highly qualified profes sionals, The immediate availability of results ccan enable the appropriate treatment to be instituted quickly. Patients’ fears can be allayed, Hower that the limitations of any test and the ificanes ofthe results are appre by the tester to avoid inappropriate inter vention or unnecessary anxiety TESTS PERFORMED AWAY FROM THE LABORATORY Table 1 shows what can be measured in a blood sample outwith the normal The most common blood test laboratory isthe determination sample, at home or in the elinie. Diabetic tor their blood lucose on a regular basis can do so at homeorat workusing one of many commer: cially available pocket-sized instruments Fg. A portable bench analyser (Courtesy of Boehringer Mannheim Gib). Figure 1 shows a portable bench ana: This instrument may be used to monitor patients’ glucose and cholesterol, nd is frequently used in many outpatient Clinics and in sereening centres, Table 2 lists urine constituents that ccan he measured sway’ from the labor tory. Ma niently measured, ‘semi- quantitatively, using est strips which are dipped briefly into a fresh urine sample. Any excess urine is removed, and the resultassessed aftera specified time by compa zewith acode on the sid The information obtained from such tests is Of the test strip container of variable value to the tester, whether patient or clinician, ‘The tests commonly performed away from the laboratory can be categorized as follows, A. Tests performed in medical or nursing settings, They clearly give valwable nd allow the practitioner the patient or family, o¢ further invest initia ations o treal- B, Tests performedintke home, shopping centre or clinical setting, They can give valuable information when properly and appropriately use C. Aleoholiests. These aresometimesused to assess fitness to drive. In clinical practice alcohol measurements needto be carefully interpreted, In the Acci xd Eme caution must be taken before one ean fully with head! injury to the effects of aleo- hol, common complicating feature in such patients, Tabi Common tess on blood periormed away from the laboratory | arse Uses when invesigating ecdgisee Ae tas sis lncoe Daets ots use eral asaase Conon Paral eaee few Neca aon ® Treapaie digs Compas orexoty Seyste ———_Oseetonot sone Puede Deaton pening Oo Date montong e Chose Caan tert ease esl Fires version, Table Tests on urine performed anay from the laboratory Anaylo Used whan vesting Kalones abet ease Pree era dscase Tasnegeon Rena dss ‘nin er disease and ude | Uctiregen anaes move Dales alis ° ree Prgracy st METHODOLOGY OF SIDEROOM TESTS Iisa feature of many sideroom tests that their simplicity disguises the use of sophisticated methodol pregnancy test method involves an ele A home nt pplication of monoclonal antibody tech: nology to detect the human chorionic gonadotrophin (hCG) which is produced by the developing embryo (Fig. 2), T testis simple to carry out; a few drops of urine are placed in the sample window and the result is shown within five minutes, The addition of the urine solubilizes a mono-clonal antibody for hCG which is covalently bound to tiny blue beads. A second monoclonal, specific foranother regionof the hCG molecule, is firmly attached in a Tine at the result window. If ACG is presentiin the sample is bound by the first antibody, formi blue bead-antibody-hCG complex. As the urine diffuses through the strip, any hCG present becomes bound atthe second antibody site and this concentrates theblue bead complex in a line—a positive result. A third antibody recognizes the ‘constant region of the first antibody and binds the excess, thus providing a control to show that sufficient urine had been added tothe test strip, the most likely form of error. GENERAL PROBLEMS ‘The obvious advantages in terms of time saving and convenience 10 both patient andclinician mustbe balanced by anumber ‘of possible problems in the use of these tests, They include: ‘© Cost, Many of these tests are expensive alternatives to the traditional methods ‘used in the laboratory, This additional expense must be justified, for example, ‘onthe basis of convenience or speed of obtaining the result '* Responsibility. The person performing the assay outwith the laboratory (the ‘operator) must assume a number of responsibilities which would normally be those ofthe Iahoratory staff, There is the responsibility to perform the assay appropriately and to provide an answer that is accurate, precise and meaningful. ‘The operator must also record the result, so that others may be able to wit (e.g. in the patient's notes), and iimegpret the result in its clinieal context ANALYTICAL PROBLEMS: Many problems under this heading will have litle to do with the assay technology but will be due to operator errors. Tests designed for use outwith the laboratory are robust but are by no means foolproof, Most operators will not be trained laboratory technicians butpatients, nurses ‘orclinicians. [fan assay is tobe performed well these individuals must be trained in its use. This may require the reading of a simple set of instructions (e.g. a home pregnancy test) or attending shor raining sessions (e.g. the ward-based blood gas analyser) The mostcommonly encountered analytical errors arise because of failure to: © calibrate un instrument ‘© clean an instrument ‘© use quality control materials ‘© store reagents or strips in appropriate conditions, All of these problems can be readily ‘overcome by following instructions care- fully. Regular maintenance of the equip- ‘ment may be necessary, and simple quality control checks should be performed. It should always be possible to arrange simple quality control cross checks with the main biochemistry laboratory. INTERPRETIVE PROBLEMS Even when analytically correct results are obtained, there are other problems which ‘must be overcome before the exercise ca be considered a success, The general appropriateness of the test must be con= sidered, If an assay is performed in an individual of inappropriate age, sex, or at the wrong time of day. or month, then the result may be clinically meaningless, Aurine sample is appied tothe test stp. set no an yes tached to bive Beads 2 LS tnd NOG-speotc i /anttodies hi J ard antivedy Urine saturates absorbent pao and begins emove along os sp + iS [Gaerne aah Boag complex, which then moves along the plate as the uine cifuses. > sy bie bead complex ‘binds te a 2nd NOG spectic antibody ined tore plate along a sagt ine. Ts ‘roducs a Bue ine onthe pat. fig Postve est ‘Excess of the monoclonal antibody buue ‘bead compton nthe urine binds toa re anvoody fering another blue ine, ‘Tis signals he tests complete, = ive result A postive result is shown by 2 Bue Ene: 2 hegatve resus show By 1 De tne Fig.2 How aprognancy test kit works. Similarly, the nature of the sample col- lected for analysis should be considered ‘when interpreting the result. Where the results seem at odds with the clinical situation, interference from contaminants (e.g. detergentsinurinecontainers) should be considered as should cross reactiv of the assay with more than one analyte (e.g. haemoglobin and myoglobin), ‘Any biochemical assay takes all these potential problems intoaccount. However, ‘with extra-laboratory testing, correct interpretation of the result is no longer the laboratory's responsibility but that of the ‘operator. ‘THE FUTURE, ‘There is no doubt that in the future, bio- chemical testing of patients outside the laboratory will become practical for many of the analytes currently measured in the laboratory. There is, however, likely tobe ‘much debate about costs and the clinieal usefulness of such non-laboratory based alyses. Case history 3 At a village fete, a local chatty group was fundraising by per- ‘forming certainside-room texts. An ‘L-year-old boy was found to have 4 blood glucose of 144 mmol. His family was concerned, and an hour later his cousin, a recently diagnosed diabetic, confirmed the ‘hyperglycaemia with his home ‘monitoring equipment, and found lycosuria +44, © What i the significance of these findings? Comment on page 152.Malnutrition is a common problem ‘worldwide, and in developed counties it is associated particularly with poverty and alcoholism. It is also encountered among patients in hospital. Various studies have shown that patients: may have evidence, not only of protein-calorie ‘malnutrition, but also of vitamin and mineral deficiencies, especially after ‘major surgery or chronic illness. Malnutrition © the layman usually ‘means starvation, but the term has a ‘much wider meaning encompassing both the inadequacy of any nutrient in the jet as well as excess food intake, The pathogenesis of malnutrition is shown in Figure 1. Malnutrition related to surgery or following severe injury occurs because of the extensive metabolic changes. that accompany these events: the ‘metabolic response (0 injury” (pp. 102-103). ‘The assessment of a patient suspected, of suffering from malnutrition is based on: story © examination « laboratory investigations including biochemistry. HISTORY Past medical history may point to changes in weight, poor wound healing for increased susceptibility 10 infection, ‘The ability to take a good dietary history isone of the most important parts of a full nutritional assessment, Taking a dictary history may involve recording in detail the food and drink intake of the patient over a 7 day period. More usually, however, a few simple questions may yield a lot of useful information about ‘person's diet. Depending on the back- ground to the problem, different quest- ions will be appropriate. For example, in the wasted patient, questions about appetite and general food intake may suggest an cating disorder such as ient presenting with a skin rash, details of the specific food groups eaten will be required to help exclude a dietary cause. In the patient at increased risk of coronary heart disease, questions on saturated fat intake may be most revealing, Lina or “Toss ot utenis Fg. 1 The development of manutriton. EXAMINATION iple_anthropometric_ measurements will include height, weight, arm circum- ference and skin-fold thickness. ‘Body mass index (weight in kg divid- ced by the height squared in metres) is a reasonable indicator of nutritional state, except when the patient is oedematous, Arm circumference is an indicator of skeletal muscle mass, while skin fold thickness is proportional to body. fat levels. In addition, general physical examination may reveal signs of mal- nutrition in the skin, nails, hair, teeth and mucous membranes, BIOCHEMISTRY ‘A number of biochemical tests are used to complement the history and examination inassessing the general nutritional status of a patient. None are completely ssitisfactory and should never be used in isolation. The most common ests include: ‘© Protein. Serum albumin concentration isawidely used but insensitive indicator “able | Clasistion of vitamin of protein nutritional status. It is affected by many factors other than nutrition, e.g. hepatic and renal diseases and the hydration of the patient, Serum albumin concentration rapidly falls as part ofthe metabolic response to injury. and the decrease may be mistakenly tributed 16 malnutrition, © Blood glucose concentration. This will bbe maintained even in the face of prolonged starvation, Ketosis develops during starvation and carbohydrate deficiency. Hyperglycaemia is Frequently encountered as part of the _metabolic response to injury ‘ Lipids. Fasting plasma iiglyceride levels provide some indication of fat ‘metabolism, but are again affected by a variety of metabolic processes. Essential fatty acid levels may be measured if specific deficiencies are suspected. Faecal fat may be measured both qualitatively and quantitatively in the assessment of malabsorption (pp. 104-105), (Asta) Sony Paar out is (Mem) eter Pasa lve or RSC rants acmaton (btn) argyle detceey Psa lees or REC gains eta actin 8. rtoane) Demis Araia Pasa es or RECAST acraton 8 Cetlon) Poricaus ancom Senn, hood cour | Fate Meaobsicarooia Saute, RBC Wt, lle out Nace Potaga nay iach ett Fat Sonne : Aetna, ness Sennen A Dobe sonic rets Sou 5-hyoncale romper rae Nouepaty Sou vain K(@hyomeratone) Detective cating Pron ines.0mgis 145mois 400mo18 Fe.2 Average adult cay requirements of vitamin. Unlike the assessment of overall status, biochemical measurements play a key role in identifying excesses or Ueficiencies of specific Components of the diet, Both blood ‘and urine results may he of value, Such assays include! * Viamins. These organic compounds are not synthesized by the body but are vital for normal metabolism. Usually they are classified by their solubility; they are listed in Table 1 and their average adult daily requirements shown in Figure 2. Some assays are available to measure the blood levels of nins directly, but often functional assays that utilize the fact that many vitamins are enzyme cofactors are used, These latter assays may help identify gross abnormalities. How- ever. to detect subtle deficiencies and the inereasing problem of excess intake, quantitative measurements are required '* Major minerals. These inorganic elements are present in the boul in quantities greater than 5g. The main nutrients in this category are sodium, potassium, chloride, calcium, phosphorus, and magnesium, All of these ure readily measurable in blood and their levels in part reflect dietary intake. ‘Trace elements. Inorganic elements present in the body in ‘quantities less than Sg are often found in complexes with proteins. The essential trace elements are shown in Figure 3. PRE-OPERATIVE NUTRITIONAL ASSESSMENT Nutritional assessment is not only necessary following surgical procedures, Patients need 10 be in good nutritional condition before an operation and the assessment should be done well in advance to allow build up of reserves before surgery (Fig, 4) Case history 40 AGS. year-old mae with motor neurone disease isadmited because of severe anorexia and weight loss. Suspecting ‘malnutrition, the house officer requests a battery of ‘ochenical ests including serum vitamin Eand selena How useful will these be in the management of this patent? : Comment on page 156. Mangan ‘copper Fluor Molybdenum ‘oan ese 25mg 15mais ‘omais soma o1amaa Selim 0G7mgd Chromizn 010mg ana Zine im 002mg 120mg 100mg Fig. Average adult daly requirements of essential trace elements. (ios aay alos Fig. 4 Selection of patients for preoperative nutritional support. Clinical note 2) Accurate measurement of height and wei ae the most important features of global hutritional assessment at all stages of life eonatal period 10 old age. These are also offen very poorly recorde patient notes. from theNutritional support may range from simple dietary advice to long-term total parenteral nutrition (TPN). In between is awhole spectrum of clinical conditions and appropriate forms of nutritional support (Fig. 1). As we move to the right, climbing the scale of severity of disease, ‘we increase the level of support and in so doing increase the need for laboratory back-up. The clinical biochemistry laboratory plays an important role in the diagnosis of some disorders that require specific nutritional intervention, ez. diabetes mellitus, iron deficiency anae- mia and hyperlipidaemia, but a much greater role is played in the monitoring ‘of patients receiving the different forms of nutritional support. Resuced ‘calories High rice Got Fg. 1 The spectrum o nutritional support WHAT DO PATIENTS NEED? Assessing the dietary needs of some patients is a highly specialized task, but some general guidelines can be considered. A balanced mix of nutrients must contain adequate provision for growth, healing and pathological losses, e.g. a draining fistula, Where patients are able t eal a mixed dict, the detailed consideration of their spe fic dietary intakes is seldom an issue. However, for those patients where the clinical team has to assume the responsibility of providing the balance of nutrients, much greater care must be taken, Eroryrequrements ~ 665 + 1 8(wophtin kg) , ” + 5.0 (height in om) Energy Patients require energy. the amount of which can be roughly calculated from the Harris-Benedict equation (Fig. 2). This formula provides the basal enersy requirements of an individual, and these must be adjusted to take account of increased requirements or losses as described above, ‘The principal energy sources in the diet are carbohy= rates and fats. Glucose provides 4 keal/g while fat provides 9 kcal/g. The entire calorie load may be admin- stered using carbohydrates, but prescribing a mixture of carbohydrates and lipids is more physiological and serves to reduce the volume of the diet. This is important in tube feeding as well as in parenteral nutrition, ray equa 6550 + 8.8 ink) +18 (ht in om) = 47 (a9 inyoar) Fg. 2 The Harrs-Benedit equation. Nitrogen HOW SHOULD THEY RECEIVE IT? Proteins and the amino acids provide dictates amino acid needs as well as Patients may be fedin the following ways: nitrogen and also yield energy at 4 the energy requirements to ensue that at feeding keal/a, Amino acids are essential both for protein synthesis and for the synthesis Of other nitrogen-containing compounds. Usually it is recommended that protein intake should constitute 10-15% of the total calorie requirement. Positivenitrogen balance can only be achieved ifthe energy imtake is at least adequate to cover the resting energy expenditure. The energy needs can be calculated from knowledge oof urinary urea excretion (Fig. 3). This protein synthesis takes place Vitamins and trace elements, Vitamins and trace elements are collect ively described as ‘micronutrients’, not Decausethey are of limited importance but because they are required in relatively small amounts. Recommended dietary allowances (RDAs) have been defined for ‘most nutrients and these are used in the make up of atificial diets ‘© tube feeding into the gut ‘ parenteral feeding. Oral feeding should be used when- ever possible. Tube feeding (Fig. 4) involves the use of small bore naso- gastric, nasoduodenal and gastrostomy tubes, Defined diets of homogeneous composition ean be continually adminis- tered. Tube feeding in this way by- passes problems with oral pathology,Urine in 2ahe and urea concentration ~ Each ire of une contains 150 mmol of wroa 02 Ries contains 300) Each molec of urea contains 2ators oN) ‘se mutiplyby2_/ ‘Convert molar to mass nts by ming) Urea ony constttes 80%. of wna N 0 rmutipy bye) 10.59 ota N . Add -29 00 onine N osses 9 laoces and skin tg Ns equivalent aa 78g protein ail Fa, 9 Urine urea measurements over 24fr may be used to assess nitrogen balanes. Case history 41 [A patient with pernicious anaer is being treated with parenteral vitamin B,;. Because she has recently been feeling tired and ‘run down’ her physician sendsasample to the clinical biochemistry lab requesting a serum B., level. ‘ Isthisthe mostappropriate way to ‘monitor the patient? Comment on page 156. @ minority swallowing difficulties (c.g. after a stroke) and anorexia. Even patients who have had gastric surgery can be tube fed post-operatively if a feeding jejunostomy is fashioned. he operation distal to the lesion. However, tube also presents mechanical problems in terms of 1 erosion, Gastrointestinal problems such as vomiting and diarrhoea, and metabolic problems can be minimized by the gradual introduction of the feeds and rarely are contraindications to enteral feeding, The problems associated with parenteral nutrition are even more severe and are discussed on pages 100-101. It should be not that the an be fed very successfully ‘age of oesophag. fast majority of patients, either orally or with enteral tube feeds MONITORING PATIENTS c and biochemical monitoring should always go hand in hand in the assessment of any form of nutritional support In some circumstances the contribution of the laboratory may be the simple measurement of blood glucose, while in other sin ‘urements and advice provided by the lab may dictate the regimen in a patient receivin parenteral Fig. 4 Patient on tube feeding in IU. Note this patient is alo being ‘entilated via tracheostomy and bas central line in place Clinical note By far the most effective route of supplying nutrients toa patient is via the gut. By using nasogastric tubes and electively inserted stoma tubes to the stomach or small intestine, only a small patients will require to be fed parenterally.: PARENTERAL NUTRITION The provision of nutrients to the body's hly complex physiological many endocrine, cells is a hi process involvin; exocrine and other metabolic functions, Total parenteral nutrition (TPN: pletely bypasses the gastrointestinal delivering processed nutrients directly into the venous blood. Its more physiological to feed patients and parenteral nutrition should only be considered once other possibilities have been deemed unsuitable. The institution of TPN is never an emei nd there should always be time for consultation and for baseline m ormed. A team approach is desirable 0. ments to be INDICATIONS FOR PARENTERAL NUTRITION Patients who are unable (© eat or absorb food adequately from the tract should be considered for parenteral ees where this, nutrition, The circumst ‘occurs include: * inflammatory bowel disease, ‘rohin’s disease * short bowel syndrome, e.g, mesenteric artery infarction. ROUTE OF ADMINISTRATION Paren following ways: n may be given in the ‘© Via peripheral veins. This route may be successfully used for a short period of 1-2 weeks, © Viaa central venous catheter. This feeding is anticipated. Central vein catheters may remain patent for years if cared for properly Althou in-patients, ma i most recipients of TPN are individuals who require long-term TPNhave successfully managed to administer TPN in the home. patients have permanent central catheters through which pre-packaged nutrition fluids are administered, us These COMPONENTS OF TPN TPN should vide complete artificial nutrition. An appropr will contain source of calories, amino acids, vitamins and trace elements (Fig. 2). The calorie est8, Pro te volume of flu nt (Be ; ‘ean? ) (Pv eer cova { “Guteency? “ ths patent recelung requ Ib song? Fig.1 Team approach to TPN, Many patients who receive TPN are given standard proprietary regimens of pre: packaged solutions. ‘mixture of glucose and lipid These have made TPN much easier, but as with any such approach in medicine there are some patients who require more tailored COMPLICATIONS Total parenteral nutrition is the most extreme form of nutritional support and cean give rise to considerable difficulties. In order to pre-empt these, consistent Fig.2 TPN preparations. Postioned ard _ careful nursing care and biochemical monitoring are required, Catheter site sepsis is in these patients, The nutrient-con Infusion fluids are, of course, also exc lent bacterial and fungal growth media, and risk of infection is further heightened by the presence of a foreign body, the catheter. Strict septic technique both in the siting of a cal and in its maintenance will serve to avoid ‘many of these problems, Misplacementofacatheter and infusion of nutrient solutions extravascularly can be very serious. Central catheters should vonstant fear attention tobe placed under X-ray control. The pos- sibility of embolism, either thrombotic or air should be easily avoided as long 28 their potential is recognized The most common metabolic com: plication is that of hyperglycaemia, Against a background of increased stress hormones, especially if there is infection, there may be marked insulin resistance and consequently an increased glucose level. The use of insulin to correct these metabolic effects is best avoided. The composition of the i.v. regimen should be adjusted if metabolic Uisorders occur. Many other biochemical abnormalities have been reported in association with TPN. These include: «hypokalaemia * hypomagnesaem * hypophosphatac hyperealeaemia acid-base disorders hyperlipidaemia If paticnts are properly monitored both clinically and biochemically during their ‘TPN these potential disorders should be avoided. MONITORING PATIENTS ON TPN In lion w bastneasesment aes fans TPN hs aust] fl (eee be a strict policy for careful clinical and biochemical monitoring of these patients (Fig. 2). This especially importantif the TPN is medium to long-term. The tests described on pages 96-97 have particular relevance here Special attention must be paid to the micronutrients in long-term TPN patients as any imbalance here may result in a single nutrient deficiency state. Such situations are increasingly rare except in Fg $ Intravenous nutiton and ts monitoring those patients relying solely on artificial diets for their nutrients, Because biochemical changes may precede the development of any clinical manifestation of a nutritional deficien: careful laboratory monitoring should be instituted. Case history 42 A S4-yearold man was admitted 1 superior mesenteric artery thrombosis. He had gross bowel ischaemia and necrosis. Subse- ently he had only 1S em of viable Clinical note aie Patients often receive ‘© What form of feeding would be lipid emulsions as part deprtpoi cin waite? of their.v. regimen. Visible o Whitassssdinett should he tinge lipidaemia in a blood sample usually suggests that the patient is tunable to clear the lipid from the plasma before commencing treatment?” Commena on page 156,‘The body reacts to all forms of noxious stimuli with an inflammatory response, This is a complex series of events which varies from mild hypersemia due to a superficial scratch to sjor haemodynamic and metabolic responses to a severe injury. Trauma isthe major cause of death in those under the age of 40 in the Western world and is the largest single cause of hospital admission. A number of important biochemical changes: take place in the traumatized individual as part of the metabolic response to injury (Fig, 1). These changes occur to some extent in all forms of injury but will only attain clinical importance when the degree of injury is severe, i. following extensive burns, multiple injuries and fractures. Major surgery or severe infection sive rise fo similar metabolic consequences. ‘The problems faced by the traumatized individual ae listed in Table |The metabolic response to injury can be thought of as protective physiological response designed to keep the individual alive until healing processesrepairthe damage thathas been done, is mediated by a complex series of neuroendocrine and cellular processes, all of which contribute tothe overall zoal—surviva. ‘THE PHASES OF THE METABOLIC RESPONSE TO INJURY ‘The metabolic response to injury has two phases. the ebb and the ‘low (Fig. 2). The ebb phase is shor and corresponds roughly to clinical shock. The physiological changes which occur here restore adequate vascular volume and maintain essential tissue perfusion, The ebb phase may progress either to death or to the flow phase, which may last from days to weeks depending on the extent of the injury. In this phase, metabolism is altered to censure that energy is available for dependent tissues at the expense of muscle and fat stores. These biochemical changes (Table 2) are mediated by the hormones comtiso, glucagonandltheeatecholamines. They ensure survival in the short term but exact a penalty from the patient, ‘Whereas loss of body fat isacceptable and easily reversed, loss of rnuscle tissue isa serious concer. The inflammation subsequent to injury o¢ infection is me- diated by paracrine regulators; eytokines such as tumour necrosis factor and the interleukins, and lipid mediators such as platelet- activating factor and thromboxane A, ‘THE ACUTE PHASE PROTEIN RESPONSE ‘The acute phase protein response leads to greatly increased de novo synthesis (principally by the liver) of a number of plasma proteins along with a decrease in the plasma concentration of some others. This response is stimulated by the release of cytokines. such asinterleukin I and 6 and tumournecrosis factor, and raised ‘concentrations ofthe hormones cortisol and glucagon, The major ‘Teble 1 Major problems faced by traumatized Individuals Beding a eton colge ~— | ‘Major ssue damage lncseased ghuconeogenesis Font an eptn | ait ciaton ress pcs Alar temperature (°C) 38 » yaaa * Pilates = | eae oe Basal oxygon consumption (nin) o i @ 3 4 6 6 7 @ t Days ae injry jy Fig. The changes in body temperature, pulse rate, oxygen consumption ‘and urinary lagen excretion which accompany injury. ‘human acute phase proteins are listed in Table 3 “The acute phase protein response isan adaptive response to disease. Is role isnot fully understood but certain aspects ean be seentobe of benefittothe individual The increases inC-reactive protein (CRP) and complement will contain and eliminate infection; increased coagulation factors will aid and prevent excess blood loss; protease inhibitors will prevent the spread of tissueneerosis when lysosomal enzymesare release by damaged cellsatthe sit of injury. The precise role of other proteins inthis response such as caeruloplasiin and serum amyloid A remains tobe established. Table 2 Biochemical changes inthe metabolic response to injury es noes eda on gh eeu nye Larrea ry co cu ean eg se asso howet in aaneh we ct mere eerie ‘mony cere pe Lends to increased amino acids which may be catsbolized to provide (used ‘Stans areca ag. ——Enoiay cra Cereacive proven tra) = Normal CAP 1 ABegmaty nn in at ‘ys oxtop 2 Patents condition Seterarating, ‘wound reapened find abssoae “rained on day S 23 CAP evel al Srspasont Days ater iriny Fg 2 The phasos ofthe matabolle response o injury. Clinical uses In practice the major use of the acute phase response is to monitor the course of the inflammatory process in the patient ‘This is done in wo ways # By measuring serum CRP.CRP ‘concentrations change very rapidly and can be used to monitor changes oon a daily basis (Fig. 3). By monitoring the erythrocyte sed- ‘mentation rare (ESR). This reflects fibrinogen and immunoglobulin con- centration, ESR changes slowly, and is used to monitor the iaflammatory process over weeks rather than day’, Table Te acute phase protein response Clinical note Antibiotic therapy for an infection, pertaps & indicated by increased CRP ‘concentration, should be started ‘only after appropriate specimens have been taken for bacteriological investigation, Days postop ‘mating good covey Fig. CRP concentration inpatient who developed an occult abscess. {allowing abdominal surgery. neonates and immunosuppressed patients, bacterial infection can be d Failure to make the diagnosis fatal consequences. In practice, a serum CRP concentration of >100 mg/l (normal <10 mg/l is frequently taken to indicate the presence of infection, STARVATION AND THE METABOLIC RESPONSE TO INJURY “The metabolic responses to injury and! to starvation are quite different. After injury the body isat war, defences are mobilized ‘metabolic activity inereasesand resources. are directed to the site of action. In starvation, the body is ina state of famine, resources are rationed and metabolic activity is limited to the minimum for survival. The two situations are quite distinet, but in the clinical situation both ccan occur together. The severely injured patientis often not fed asa priority. Where starvation and the metabolic response to injury occur together, a good clinical ‘outcome becomes less likely. Iaseriously ‘traumatized patient is also nutritionally depleted he or she may be exposed to: Case history 43 A 28-year-old man was admitted 1 the intensive therapy unit after a setiousroadtrafficaccidentin which ‘he sustained multiple injuries. After intial resuscitation and surgery to hisinjurieshe wasconsidered stable ‘but in coma, ‘© What is the role of biochemical measurements in this patient's ‘management? Comment om page 156. ‘© immunosuppression © decreased wound healing # delayed tissue repair © muscle weakness, All of these work together to prolong convalescence, increase morbidity and cloud the outeame, ‘The importance of prompt nutritional support for severely injured patients can- not be overemphasized. In the catabolic state, large amounts of energy and amino acids are required to replace the losses, ‘The nutritional workup and assessment appropriate in the trauma patient sre diseussed on pages 97-99. In addition to ensuring adequate hutrition, some investigators believe that other therapies may enhance patient recovery following trauma, burns or severe infection. Strategies which reduce the inflammatory response and enhance protein synthesis and tissue repair are being evaluated.PHYSIOLOGY OF DIGESTION AND ABSORPTION Having been initially broken down in part by cooking and mastication, food is assimilated by the body by the com= bined action of 10 processes: digestion and absorption. The major nutrients (carbohydrate, protein and fat) are broke down enzymatically to low molecular weightcompounds. Digestive enzymes ‘are secreted by the stomach, pancreas ‘and small intestine The transport of the products of digestion into gut epithelial cells and from there to the portal blood is termed absorption. The absorption of some nutrients is passive while others require active transport. MALABSORPTION Failure of digestion is properly called maldigestion. The term ‘malabsorption’ describes impairment of the absorptive ‘mechanisms, but in practice is used to encompass both disorders, Malabsorption isacondition which canoceurat any stage of life from a variety of causes (Fig.l). The clinical effects of malabsorption result from the failure to absorb nutrients. The major consequences of generalized malabsorption arise from inadequate energy intake Which results in weight loss in adults and growth fa Diagnosis In suspected malabsorption a detailed dietary history is essential to establish eating patterns and habits. Endoscopy and biopsy are by far the most important tools available for the investigation of gastrointestinal tract disorders. They allow macro- and microscopic study of the intestinal mucosa and are the standard techniques used to diagnose major causes ‘of malabsomption such as coeliac disease. cal tests are of assistance when ‘abnormal anatomy of the bowel and motility. Though not usually investigated, itis important to assess the state of teth and gums, and the adequacy of salivary secretion, as they play an important role in initiating digestion, Provided that dietary input isadequate, the presence of malabsorption from the small bowel will often be indicated by changes in the faeces, in particular by diarrhoea. Diarrhoea due tomalabsorption cean be assumed only if other causes, such asinfection and laxative abuse, have been Fp. 1 Causes and consequences of malabsorption. ‘excluded. Laxative abuseis an important diagnosis which may he missed, A laxative sereen is readily performed in cases of suspected abuse, In the case of fat malabsorption, the faeces will contain fat which can be detected by microscopic examination of the stools or by quantitative analysis, Where small molecules such as. mono- xccharides are not absorbed, they exert an osmotic effect which will prevent water ‘absorption in the large intestine, giving rise toa large volume of watery stools, Biochemical investigations Laboratory tests in the investigation of gastrointestinal disorders fall into one Of two groups: tests which identify malabsorption and tests of pancreatic funetion. The most frequently performed tests are outlined below. There is little agreement amongst investigators as to the best tests (0 use. ‘Tests of malabsorption ‘© Paccal fat. The presence of fatty stools is an important sign of Muscle woakness [hat aie Poriperal neuropathy {tam deterency) {Canto hypstuinsemi) :malabsorption, Faecal specimens over five day period may be collected and total fat content measured, © Faecal microscopy. The presence of fat globules can be observed directly. ‘© Butterfat rest. Chylomiezons detected inthe plasma of patients after a standard fat load indicate that some ligestion and absorption bas. urred, © “Cuirigiyeeride breath test. An oral dose of radiolabelled triglyceride (e.g: MC-triolein in which the Fatty acid contains the label) is absorbed and ‘metabolized. The "CO; in expired air is a measure of the effectiveness of digestion and absorption. ‘© Xplose absorption rest, Serum ‘measurements of this 5-carbon sugar are made after an oral dose and provide an indication of intestinal ability to absorb monosaccharide ‘Tests of pancreatic function © Lundh test. Duodenal contents are collected after a meal and the activity of apaneteaticenzyme, suchas trypsin or amylase, is measured‘Tele 1 Adaltiona biochemical tests used inthe investigation of gastrointestinal disease sed ey ptr wth Hebe pi neten which sera ‘sorted wih posi uber ease Assesses bac mergoxtin ie tsing Dee daccaries ray be ued cat speck cons eng atecs sh a eta sae nrc nce utn ets ot ne assesnert ol parca on ‘may nese rs ‘Siegel nt polars ae used assess mos ean by ‘meautng Polen sat ean ol ne * Secretin test. An intravenous injection of secretin stimulates. pancreatic hich isassessed by aspiration of duodenal contents and measurement of trypsin or amylase activity © Pancreotaury! test. Fluorescein dil aurate is hydrolysed by cholesterol esterase in pancreatic secretion. The ‘water-soluble Tuoreseein is absorbed and excreted in urine where its TTuorescence gives an indirect measure of pancreatic function Other tests which may be employed in the investigation of makibsorption and _stointestinal disease are shown in Table | eralized mal- absorption occurs for a number of reasons. These include: ‘Inadequate digestion, which is seen in chronic pancreatitis as a consequence of insufficient pancreatic enzyme release, Case history 44 A @9-year-old woman, sho had made an excellent recovery after ‘ocal excision of a breast rumour 8 ‘years previously, presented with ‘weight loss, bone tenderness and weakness. Her symptoms had developed oyeranumberof months. Her family were concerned that she “was not caring forhersel noreating adequately. There was no clinical ‘evidence of recurrence of breast cancer, LFTs showed only an Fp recaiatrc tris C0 © What other biochemical tests ‘would be of assistance in making «diagnosis? ‘Comment on page 156. Assesses van B ston ‘© Inadequate intestinal mucosal surface (Fig, 2) as oveurs in coeliac disease. © Inflammation affecting the mucosa, sub mucosa and frequently the entire bowel structures as occurs in Crohn's disease. © Infections of the bowel causing intestinal hurry. These may be acute as in salmonella, or chronic as in tropical sprue, © Abnormal bowel anatomy or insut jent bowel. This may occur after repeated surgery for chronic disorders such as Crohn's disease or after bowel infarction and removal of the necrotic bowel Malignant disease is not usually associated with malabsorption unle there is abnormal bowel motility whi allows bacterial flora to proliferate, or if the tumour secretes a hormone such as VIP, which causes diarrhoea (p. 132), Inadequate bite salt secretion occurs in many forms of liver disease and gives rise to fat malabvorption. Patients may Present with the clinical features of generalized malabsorption on top of the underlying liver disease. Inadequate fat absorption is associated with impaired absorption of vitamin D (a fat-soluble n). In the absence of adequate paticnts may become vitamin D deficient and unable to absorb @ Clinical note jis with mal- absorption recognise that ‘certain things in their diet—usually fatty foods —eause diarrhoea. They avoid these foods and reduce their fat intake. As a result, faecal fat exeretion mnay be normal because of this low dietary fat intake ope sez Mucor folds Incossod to 1.0m? Mucosal vith ‘Siriace ston inereased'o 10m? Fg? Etfects of mucosal structure on absorptive surface area of the small intestine ccaleium from the gut. Thismay lead wo the development of osteomalacia or rickets, The malabsorption of specific sub- stances tends to occur in certain well- fined conditions such as vitamin By: ‘malabsorption which leads to pernicious nia. This occurs when intrinsic factor production is lost due 10 gastric ‘mucosal atrophy. Patients may also have inherited deficiencies of intestinal ccharidases such ay lactase which cause malabsorption when the patient drinks milk or eats milk products. This is known as lactose intolerance.Tron is an essemtial element in humans, being the central ion in haem, the non: protein constituent of haemoglobin (Fig. 1). Haemoglobin is responsible for the transport and delivery of oxygen from the lungs to the tissues. Iron deficiency causes a failure of haem synthesis, leading to anaemia which results in tissue hypoxia. Haem is alsoa constituent of myoglobin in muscle issue, Cytochromes and many other intra- cellular enzymes also contain iron, but are largely unaffected in all but the most severe cases of iron deficiency. M— Motyt Vv — vinyl P — Propionate ig 1 Stuctre of haem. IRON PHYSIOLOGY ‘There are 50-70 mmol (3-4 2) of iron in the body. Three-quarters are present as the molecular complex with haem. Most ‘of the remainder i in tissue stores bound 10 the iron storage proteins, ferritin and haemosiderin, Less than one per cent of total body iron isin the plasina, where itis associated with the iron-binding protein, transferrin, Body iron is efficiently reutilized, Dietary intake is about 0.35 ‘mmol (20 mg) per day. The factors which regulate intestinal absorption of irom are poorly understood (Fig. 2) ‘Serum iron concentrations vary with ‘age and sex. Normal adult concentrations fre 18-45 mol/l in males and 14-32 |umol/ in females. In both groups there is ‘a marked eiteadian rhythm in serum icon ‘concentration Iron is transported in plasma bound t© the specific glycoprotein, transferrin, cach molecule of which binds two Fe ions together with bicarbonate as an anion, A number of factors can lower serum iron independently of iron status, These Include the acute phase response to infection, trauma and malignant disease LABORATORY INVESTIGATIONS OF IRON DISORDERS Various laboratory investigations are helpful in the diagnosis of iron deficiency and iron overload: © Serum iron determinations are of limited routine valuc, being of most assistance in the diagnosis of iron ‘overload and acute iron poisoning. * Total iron binding capacity (TIBC) ‘of a serum specimen isan indirect measure of transferrin concentration, although in some laboratories trans” ferrin can be measured directly. Normally transferrin is around 30% saturated with iron, When this falls 10 Jess than 15%, irom deliciency is likely and some degree of clinical effect ean be expected. A high percent saturation ig the most sensitive marker for iron overload. Like serum iron, transferrinis decreased as part of the acute phase response. Protein-energy malnutrition also decreases transferrin synthesis, by the liver and hence its serum concentration, © Serum ferritin is the best indicator of body iron stores, Itis normally greater than 12 yg. The acute phase response to infection, injury or malignant disease can result in increases in serum ferritin, making the diagnosis, of marginal iron deficiency difficult in these circumstances, © Red cell protoporphyrin is usually less than 1 jimol per litre of red cells. This, ron inte “2omgiéay -0.35mmatey hhaem precursor is markedly increased in iron deficiency. Levels are also affected in porphyria and by exposure to inorganic lead compounds, IRON DEFICIENCY Iron deficiency anaemia is the common est of all single-nutrient deticiences, Millions of people worldwide are affected, with seriously impaired quality of life and work efficiency, The principal ceauses are chronic blood loss and poor dietary intake of bioavailable iron, e.2. the uptake of inon from the diet ean be decreased by a number of dietary constituents such as phytic acid and fibre, Iron deficiency can also occur in coeliac disease and other intestinal dis- ‘onlers where malabsorption is a feature. Inall eases of iron defieieney anaemia itis important to diagnose the underlying condition, especially malignant disease, the presence of intestinal parasites or any other intestinal pathology which may cause chronic blood loss. Iron deficiency may also develop during pregnancy. even in well-nourished women, due to the increased iron requirements of the developing fetus. Iron deficiency anaemia develops in three stages L.A depletion of iron stores with no functional impairment, Serum ferritin concentrations of less than 12 g/l ccontiem this. Deficient erythropoiesis. Hacmoglobin levely remain normal, but red cell protoporphyrin is increased. The synthesis of transferrin is increased and percent saturation is decreased, Body distribution 00% exretod Fg 2 fon balance.Fi 9 Photomicrograph t an ron deficient blood fm. When 15% or less saturation is reached, work capacity is impaired. 3. ron deficiency anaemia. There is lo haemoglobin resulting in a microeyt hypochromic anaemia (Fig. 3). Low ble iron is seen in bone marrow, Only in the late stages of iron de- ficiency anaemia are low concen- trations of serum iron observed. Treatment Oral therapy with iron salts is tused to treat iron deficiency anaemia. It can take up to 6 months to replete the body stores. Compliance is a problem since side effects such as nausea, dia- rmioea and other intestinal complaints may be lessened if the iron salts are taken with food. A combination of iron and folate is widely prescribed during pregnancy. ncountered. These are all IRON OVERLOAD Since there is no major mechanism for excretion of iron except by cell desquamation and occult blood loss, iron overload is always a possibility when iron therapy is prescribed. Iron overload may also be caused by chronic blood transfusions. Two other con- ditions may be encountered: haema- cchromatosis and iron poisoning. & Clinical note A microcytic hypo- cchromie anaemia, with the absence of stainable iron inva bone marrow biopsy, are the best diagnostic indices of established iron deficiency. Plasma ton fovea oC Haemachromatosis, Haemachromatosis (bronze diabetes) is a relatively common inherited disease ‘characterized by increased iron absorption ‘and iron deposition in various organs Which leads to fibrosis and organ failure, The clinical presentation varies widely depending upon dietary iron intake, alcohol abuse or the presence of hhepatotoxins, Women are less severely affected than men, being protected by physiological loss of iron during ‘menstruation and in pregnancy. Whole body iron content may be increased to ten include chronic and, in extreme eases, skin pigmentation, diabetes mellitus, hypo gonadism, liver cirthosis and hepatoma, Serum iron is increased, with almost ‘complete saturation of transferrin, ‘The Case history 45 A 42-year-old woman presented ‘witha history ofinreasing lethargy, Aizziness and breathlessness. She had brite hair and nails. She complained of heart palpitation on ‘exercise and reported particularly: heavy petios. Biochemical inses- tigation revesled the following results: Serum ion ‘Transferrin saturation 10% Ferritin Sieh + What isthe diagnosis and what other investigations should have ‘been done first? Comment on page 157. 4 ym 24 Tine ateringeston hours) Fi. Theetfect of desfrioxamine on ron excretion in overdose. serum ferritin is markedly increased 10 greater than 500 g/l. Chronicironoverload ‘susually treated by repeated venesection, Iron poisoning Accidental ingestion of iron tablets by children is common and may be life threatening. Symptoms include nausea and vomiting, abdominal pain, diarshoca and haematemesis. hypotension, liver damage and coma can result, Serum iron is increased and transferrin is saturated (>70%). ‘Acute jnon poisoning is treated by’ chelating the iron inthe stomach and ia the plasma with desfertioxamine (Fig, 4). The chelated iron is excreted in the urine as @ deep orange-coloured corm plex. This should not be confused with myoglobinuriaCOPPER Copper is an essential trace metal which isa component of a wide range of intra cellular metalloenzymes, including cyto- cchrome oxidase, superoxide dismutase, tyrosinase, dopamine hydroxylase and Iysyl oxidase, Most of the copper in plasima is associated with the specific ‘copper-binding protein, caeruloplasmin. COPPER PHYSIOLOGY ‘About 50% of the average daily dietary copper of around 25 jumol (1.5 mg) is absorbed from the stomach and the small intestine (Fig. 1). Although copper is present in most foodstuffs, there is ‘evidence that not all modern diets contain sufficient copper, especially when large amounts of refined carbohydrate are ‘consumed. Absorbed copper is trans- ported to the liver in portal blood bound {o albumin and is exported to peripheral tissues mainly bound to caeruloplasmin and, 10a lesser extent, albumin. ‘Copper is present in all metabolically active tissues. The highest concentrations ‘are found in liver and in kidney, with significant amounts in cardiac and skeletal muscle and in bone. The liver contains 10% ofthe total body content of 1200 pmol (80 mg). Excess copper is excreted in bile into the gut, and the Faceal copper output (12.5 pmol/24 h) is the sum of unabsorbed dietary copper and that re-exereted imto the gut, LABORATORY ASSESSMENT ‘A number of different investigations are needed to diagnose disorders of copper ‘metabolism. These are ‘© Serum copper. Normal concentrations are usually between 10-22 mol/l, of which 90% is bound to eaeruloplas- ‘min, Total copper concentration may vary either due to changes in copper itself or to changes in the concentra- tion of caeruloplasmin. © Serum caeruloplasmin. ‘The normat adult levels are 200-600 mg/l. Caerulo- plasmin is increased greatly in the acute phase reaction, and in some cases may be so high as 10 raise the total ‘copper concentration to 30-45 jumol/I Caeruloplasmin levels may be helpful in the interpretation of serum copper concentrations. ‘© Urinary copper. Notmmal exeretion is <10umol24h, Dietary copper 12.5umotay Fg. Copper balance, COPPER DEFICIENCY Both children and adults can develop symptomatic copper deficiency. Pre~ ‘mature infants are the most susceptible since copper stores in the liver are down in the third trimester of pregnan In adults, deficiency is usually found following intestinal bypass surgery or in patients on parenteral nutrition. Symptoms range from bone disease to an iron-resistant microeytic hypo- chrome anaemi COPPER TOXICITY Copper toxicity is uncommon and is ‘most usually due to administration of copper sulphate solutions. Oral copper sulphate may lead to gastric perforation, Serum copper concentrations may be greatly elevated. Copper is toxie to many organs, but renal tubular damage is the major concern, Treatment is by chelation with penicillamine, INBORN ERRORS: OF COPPER METABOLISM ‘There are two inborn errors of copper metabolism: Menkes syndrome and Wilson's disease. ‘Menkes syndrome Menkes syndrome isa very rare but fatal condition which presents in infants as growth failure and mental retardation, with lesions of the major blood vessels and bone disease, A characteristic sign is ‘steely hair’ (pilo tort) Wilson’s disease All adolescents or young adults with ‘otherwise unexplained neurological or hepatic disease should be investigated for Wilson's disease, since this condition is fatal if not diagnosed and treated. Symptoms are a result of copper depos tion in liver, brain, and kidney. Copper deposits in the eye can sometimes be seon as a brown pigment around the iris (ihe Kayser-Fleischer ring). ‘The inherited defect in Wilson's disease is believed to be in the gene coding for an enzyme involved in ‘copper excretion into bile and reabsorp- tion in the kidney. Urinary eopper excre- tion is high and serum concentrations ow (Table 1). Just what causes the low cacruloplasmin concentrations in these patients remains unclear. Confirmation is sasurement of copper in a liver h is usually greater than patients with the disease, Clinical note In the general & en songs ae Rind arse inc ool a Cosco pe one nnphote lt ke wey alo ipsam Se