CHAPTER 8

Renal Function Tests

Objectives
Upon completion of this chapter the student will be able to:

• Define terminologies applied in the renal function tests

• Discuss the anatomy, physiology and Pathophysiology of the

renal system

• Define the non protein nitrogenous (NPN) compounds

• Discuss about, NPN compounds mainly of Creatinine, urea,

and uric acid
Objectives
• Explain about laboratory diagnosis of Creatinine, urea, and
uric acid

• Discuss renal clearance tests of creatinine, and other renal

clearance tests

• Calculate renal clearance tests result, and urea creatinine

Outline
• Definitions of important terminologies

• Anatomy, physiology and pathophysiology of the renal system

• Non- protein nitrogenous (NPN) compounds

• Urea and Blood urea Nitrogen (BUN)

• Creatinine

• Uric acid

• BUN/creatinine ratio

• Clearance tests
Definitions
• Non protein nitrogenous (NPN) substances: are end products
of metabolism that contains nitrogen
• Azotemia: An excess of urea or other nitrogenous compounds
in the blood
• Anti diuretic hormone (ADH): is a posterior pituitary gland
hormone, important for reabsorption of water from the kidneys
• Diabetic insipidus: A disorder associated with secretion and
metabolism of anti diuretic hormone (ADH), manifested by
excessive urine production
• Gout: Group of disorders of purine metabolism
Cont.’…
• Nephron: functional units of kidney

• Renal failure: Acute or chronic decline in renal function

• End stage of renal disease: a condition which renal function is

in adequate to supply life

• Glomerulus's filtration rate: a measure of function of nephrons,

particularly creatinine and urea filtration rate from glomerulus
into bowmans capsule per millimeters per minute

• Renal clearance: The volume of plasma from which a given

substance is cleared completely by the kidneys per unit of time
Major Structures of Urinary System
• Kidney

• Ureters

• Bladder

• Urethra
Kidney Functions
• Filtration of small molecules

• Reabsorption of essential substances

• Secretion into urine from blood stream

• Excretion

• Hormonal regulation: erythropoietin, ADH, aldosterone

• Homeostasis

 Regulation of acid-base balance, water, electrolyte

Renal anatomy
Representation of a nephron and its blood supply.
Major Components of Kidney
 Nephron
1. Arterioles:

 Afferent

 Efferent

2. Glomerulus

 Bowman’s capsule

3. Tubules:

 PCT, Loop of henle,

DCT, Collecting tubules
Renal physiology
• Three basic renal processes
 Glomerular filtration
 Tubular reabsorption
 Tubular secretion

FIGURE . Renal processes of filtration, reabsorption, and secretion.

Glomerular Filtration
• Non-selective filtration across semi-permeable membrane

• Low molecular weight substances will pass into the urine

• Filtrate lacks large molecular weight substances such as

protein, protein-bound substances, cells

• GFR: ~120 mL/min

 Defined as the volume of fluid that is filtered across the

glomerular capillary membrane per minute
 What gets filtered in the glomerulus?

• Freely filtered • Some filtered • None filtered

 H2O  2 -  Immunoglobulins
 Na+, K+, Cl-, microglobulin  Ferritin
HCO3-, Ca++,  1-  Cells
Mg+, PO4, etc. microglobulin
 Glucose
 Albumin
 Urea
 Creatinine
 Insulin
Then what happens?

• If 200 liters of filtrate enter the nephrons each day, but only 1-
2 liters of urine result, then obviously most of the filtrate
(99%) is reabsorbed

• Reabsorption can be active or passive, and occurs in virtually

all segments of the nephron
Tubular Reabsorption
• Passive vs. active transport mechanisms
 Conserves water and essential substances
• Water: always passively reabsorbed
• Glucose, HCO3-, amino acids, Na+, K+, : active transport
Reabsorption from glomerular filtrate
% Reabsorbed
Water 99.2
Sodium 99.6
Potassium 92.9
Chloride 99.5
Bicarbonate 99.9
Glucose 100
Albumin 95-99
Urea 50-60
Creatinine 0 (or negative)
Tubular Secretion
Eliminates waste products not filtered by glomerulus
• Drugs that are protein-bound (protein remains in bloodstream)
• Organic waste:
 Urea
 Creatinine
 Uric acid
• Uromodulin protein secreted into tubules
• Acid-base regulation: H+, HCO3-
Renal Threshold
 Defined as the plasma concentration of a substance that when
exceeded, the kidney tubules will not reabsorb any more into the
bloodstream, resulting in the substance being excreted into the
urine
 Substances are reabsorbed into the bloodstream dependent upon
their blood concentration and the body’s needs
 When the plasma concentration of a substance is higher than a
certain ‘threshold value’, reabsorption of the substance is no
longer possible
 The substance is then spilled into the urine
 Example: glucose renal threshold is ~160-180 mg/dl
Renal pathophysiology
• Renal pathophysiology deals with the abnormal physiology of
the renal system

• Signs and symptoms of renal failure

 Nausea, vomiting, edema, pain, shock, Micturia,

 Urine volume change, urine composition change….

• Types of renal failure

 Acute renal failure

 Chronic renal failure

 Acute renal failure (ARF)
• Acute renal failure (ARF)

 Is a syndrome characterized by rapid decline in glomerular

filtration rate (hours to days)

 Laboratory results show electrolyte, acid-base, and fluid

imbalances

 Retention of nitrogenous waste products

 When causes removed, recovery may occur with days and

weeks
Causes, agent, and progress parameters of acute renal failure
Cause Agent Test and progress
parameters
Prerenal
 Hypovolemia  Trauma, burns, surgery
 Nephrotic syndrom, sepsis;, shock
Measurment of
 Decreased plasma volume elecrolytes,
 Congestive cardiac failure,
 Decrease cardiac output pulmonary embolism acid base, urine
 Renovascular obstruction  Atherosclerosis, stenosis volumes,
 Interference with renal  ACE inhibitors, cyclosorin
NPN blood and
autoregulation
urine concentration.
Renal
 Glomerular and small  Aggressive glomerulonephrities
vessels disease  Infection, infiltration, drugs,
 Interstitial nephritis toxins
 Tubular lesions  Postischemic, nephrotoxine,
hypercalcemia
Post renal  Prostatism, neurogenic bladder
 Bladder outflow  Stones, blood clot, tumors,
obstruction radiotherapy,retroperitoneal
 Uretric obstruction fibrosis
Chronic renal disease (CRD)
• Chronic renal disease (CRD)

 Is a pathophysiologic process with multiple etiologies

 Resulting in the unstoppable attrition of nephron number and

function

 Frequently leading to end-stage renal disease (ESRD)

 Renal diseases
• Glomerular disease

• Cystic renal disease

• Diabetes nephropathy

• Renal calculi

• Toxic nephropathy

• Obstructive uropathy

• Miscellaneous renal diseases

Non-Protein Nitrogen Compounds (NPN)
• These are compounds that contain nitrogen, but are not
proteins

• It include end products of metabolism

• Kidneys act to excrete metabolic waste into urine

 Amino acids
 Ammonia
 Blood urea nitrogen (BUN)
 Creatinine
 Uric acid
• Measurement of these compounds in plasma is useful for
assessment of kidney function
28
Non-Protein Nitrogen Compounds
• Include >15 compounds

 Amino acids

 Ammonia

 Blood urea nitrogen (BUN)

Protein  amino acids  ammonia  urea

 Creatinine
Muscle breakdown product

 Uric acid
Nucleic acid catabolism
Urea
• Highest concentration of all NPN’s in the blood and urine

• Synthesized in the liver from CO2 and the ammonia from the
deamination of amino acids in the reactions of the urea cycle

• Major excretory product of protein metabolism

• Readily filtered from the plasma by the glomerulus

• 40% is reabsorbed by passive diffusion

 Reabsorption depends on urine flow rate and degree of hydration

• <10% are excreted through the GI tract and skin

Urea (BUN)
• Formation and excretion

 Synthesized in the liver: ammonia  urea

Protein  amino acids  ammonia [LIVER]  urea

• Conversion of ammonia to urea is last liver function to fail in

end stage liver disease

Plasma ammonia levels rise

The urea cycle
UREA
• Plasma concentration depends on:

 Renal function

 Protein content of the diet

 The amount of protein catabolism

 Liver function

 State of hydration

33
Disease Correlations
• Azotemia - an elevated concentration of urea in the blood

• Uremia or Uremic syndrome - a very high plasma urea

concentration accompanied by renal failure

• Conditions causing elevations of plasma urea are classified

according to cause into three main categories:

 Pre-renal Azotemia

 Renal Azotemia

 Post-renal Azotemia
34
Disease Correlations
 Pre-renal azotemia is caused by:
Reduced renal blood flow
 CHF
 Shock
 Hemorrhage
 Dehydration
Protein metabolism

 A high-protein diet

 Increased protein catabolism

35
Disease Correlations
 Renal causes
 Acute and Chronic renal failure

 Glomerular nephritis

 Tubular necrosis, and other intrinsic renal disease

 Post-renal azotemia

 Obstruction of the urine flow in the UT by renal calculi

 Tumors of the bladder or prostate, or

 Severe infection

36
Urea (BUN)
Decreased urea (BUN):
 Decreased protein intake (leads to decreased urea formation)
 Malnutrition
 Decreased liver function (decreased conversion of ammonia
to urea)
 Increased protein synthesis- during late pregnancy and
infancy
o Not a good test for GFR:
 Influenced by diet, liver function
Urea laboratory diagnostic techniques
1. UV enzymatic (indirect) method

2. Colorimetric (direct) methods

– Bertholet method

– Diacetyl monoxime method

– Neslerization method
 Specimen
• Serum, plasma, whole blood, and urine

• A protein-free filtrate of whole blood and based on measuring

the amount of nitrogen

• NaF in high concentration inhibits the enzyme urease

• Thymol crystal should be added to the urine and serum to

inhibit bacterial decomposition of urea
Methods of measurement Urea (BUN)

1. Enzymatic methods

 Called ‘indirect’ methods because these methods measure

the amount of ammonia ‘liberated’ from the urea molecule
present in the sample

 So urea is not directly measured (it is indirectly measured

as ammonia; amount of ammonia is related back to the
amount of urea present)

 Enzyme: urease
Enzymatic methods…
• Enzymatic urease method (indirect)

• Glutamate dehydrogenase
NH4+ + 2-oxoglutarate + NADH Glutamic acid + NAD+ + H+

• The amount of ammonium ion produced is directly proportional

to the amount of urea

• Spectrophotometric measurement of NADH  NAD (ABS at

340 nm)

• Disadvantage: endogenous ammonia will interfere

Chemical methods of Urea measurement

• Called ‘direct’ methods because urea in sample reacts directly

with reagent causing a color change that is
spectrophotometrically measured

• Color reagent: diacetyl-monoxime

• Advantage: endogenous ammonia does not interfere

 Diacetyl monoxime method
• The method with diacetyl monoxime is the only method of urea
determination where the urea molecule reacts as a whole
• Principle:
 Urea reacts directly with diacetyl monoxime under strong acidic
conditions to give a yellow condensation product

 The intensity of the yellow color is directly proportional to the

concentration of urea in the sample

 The reaction is intensified by the presence of ferric ions

and thiosemicarbazide

 The intense of red color formed is measured at 540nm/ yellow green

filter

 Specimen : serum, plasma, urine

Bertholet method
• Principle:
 Urea is split into ammonia and carbon dioxide by the action of urease

 The ammonia then reacts with alkaline hypochlorite and phenol in the
presence of a catalyst-sodium nitroprusside

 The resulting product is indophenol (a blue color) and the concentration

of ammonia is directly proportional to the absorbance of indophenol
and the abs read at 560 nm and compared with standard

• Specimen: serum, plasma, urine and other body fluids

• Oxalates and fluoride ions inhibit the activity of urease

 Cont.’….

• Calculation
Csample = Asample X Cstd or Csample= ∆A X F
Astd
• Source of error

 Hemolysis

 Lipemic sera

 Ammonia from environment

Neslerization method
• Principle:

 The sample is mixed or incubated with urease at 37oc

 Just after enzymatic action is completed, the specimen is deproteinzed

with tungistic acid or TCA

 The protein-free filtrate is treated with Nesler’s reagent, which react

with NH+4 forming a yellow-colored derivative

 The intensity of the yellow color is read at 470 nm and directly

proportional to the concentration of urea in the sample

• Specimen: serum, plasma, whole blood, urine

 Specimen collection/handling
• Urine
 Timed collection preferred

 Must be diluted prior to measurement

 1:10, 1:20

 Stability: up to 1 week stored in refrig when pH<5

 Some bacteria are able to hydrolyze urea to ammonia resulting in falsely

decreased urine urea levels

 As ammonia increases in the urine, the pH becomes more alkaline

• Serum or heparinized plasma

 Stability: up to 24 hours at room temperature; 1 week at 2-4oC

Limitations in Urea Methods
• Sources of error

 Hemolyzed or Lipemia

 Ammonium heparin cannot be used for methods using

urease

 Fluoride and citrate

 Urea and BUN

• U= Urea

• Blood Urea Nitrogen = BUN

• Urea contains 2 nitrogen atoms: 28 g nitrogen/mole of urea

• BUN x 2.14 = urea

50
Blood Urea N (BUN)
Reference Range:
• For adults (Serum/plasma)……………….. 6-20 mg/dl
• New borne up to one week( Serum/plasma)……… 3- 25mg/dl
• Adult over 60 (Serum/plasma) ……………………..8-23mg/dl
• Urine, 12-20 g/24hrs
Q1. Convert 22 mg/dL BUN to urea mg/dL

BUN 22 x 2.14 = Urea 47 mg/dL

 Creatinine
• Creatine synthesized primarily in the liver from arginine, glycine,
and methionine
• Transported to other tissue, such as muscle, where it is converted
to phosphocreatine and serves as a high-energy source
• Creatine phosphate loses phosphoric acid and water to form
creatinine which passes into the plasma
• Creatinine
 Is released into the circulation at a relatively constant rate
proportional to an individual's muscle mass
 Removed from the circulation by glomerular filtration and
excreted in the urine
Clinical Chemistry 52
Creatinine….
 Creatinine…
• Creatinine is a non-protein waste product of creatine phosphate
metabolism by skeletal muscle tissue
• Creatinine is freely filtered
• The serum creatinine level depends on the Glomerular Filtration
Rate (GFR)
• Renal dysfunction diminishes the ability to filter creatinine and
the serum creatinine rises
• The serum creatinine is a better indicator of renal function than
either that of BUN or uric acid because it is not affected by diet,
exercise, or hormones
 Creatinine…
• Creatinine production is continuous and is proportional to muscle
mass

• Measuring serum creatinine is a useful and inexpensive method

of evaluating renal dysfunction

• If the serum creatinine level doubles, the GFR is considered to

have been halved

• A threefold increase is considered to reflect a 75% loss of kidney

function
 Creatinine…
• Plasma creatinine concentration depends on:

 Relative muscle mass

 Rate of creatine turnover

 Renal function

• Concentration in the blood is reasonably stable and constant

Clinical Chemistry 57
Clinical Significance of creatinine
• The serum creatinine is elevated whenever there is:

 A significant reduction in the glomerular filtration rate or

 When urine elimination is obstructed

• The kidney reserve is such, however, that about 50% of kidney

function must be lost before a rise in serum creatinine can be
detected

58
Increased serum creatinine
• Impaired renal function

• Chronic nephritis

• Urinary tract obstruction

• Muscle diseases such as gigantism, acromegaly, and

myasthenia gravis

• Congestive heart failure (CHF)

• Shock

59
Decreased creatinine

• The elderly, persons with small stature, decreased muscle mass

• Muscle atrophy can also result in decreased serum creatinine

level

• If muscle atrophy is suspected, assessment of serum creatine

kinase, an important enzyme necessary for normal muscle
function, is done

60
Methods of measurement of Creatinine
• Chemical method: Jaffe reaction
• PRINCIPLE: Creatinine reacts with picric acid in alkaline
solution to form a red-orange chromogen
Creatinine + alkaline picrate  Janovski complex
(yellow) (red-orange color)
• Lacks specificity: modifications to the method have been made
 Protein free filtrate
 Kinetic measurement
• Less expensive, fast and easy = popular method
Limitations
• Sources of Error:

 Falsely increased results with high levels of: protein,

ascorbic acid, ketones, glucose, pyruvate and uric acid

 Falsely decreased results: bilirubin

 Specimen hemolysis or lipemia

Methods of measurement of Creatinine
• Enzymatic method: creatininase - H2O2
Creatininase
Creatinine + H2O Creatine

Creatinase
Creatine +H20 Sarcosine + urea

sarcosine oxidase
Sarcosine + O2 + H2O formaldehyde + Glycine + H2O2

peroxidase
Oxygen accptor + H2O2 Colored products

 Less interference when compared to Jaffe reaction

Creatinine Specimen collection/handling
• Serum or heparinized plasma

 Avoid hemolysis

 Avoid lipemia

 Stability: one week at refrigeration temps

• Urine

 Time collection preferred; random acceptable

 Stability: up to 4 days in refrigeration

 Longer when frozen

Creatinine Reference Range
• Serum Adult male: 0.6-1.1 mg/dl

Adult female: 0.5-0.8 mg/dl

Child: 0-0.6 mg/dl

• Urine Male: 800-2000 mg/24hr

Female: 600-1800 mg/24hr

• A panic value for creatinine is 7.4 mg/dl in non-dialysis

patients (IFCC value)
BUN/Creatinine Ratio
• Used by clinicians to differentiate causes of azotemia:

 Pre-renal

 Renal

 Post-renal
Calculated= Serum BUN (mg/dl)
Serum creatinine (mg/dl)
• Normal ratio: 10-20 with majority around 12-16

66
BUN/Creatinine Ratio

Tend to be caused by pre-renal

conditions:
Increased ratio with
 Congestive Heart Failure
increased BUN,
normal creatinine  Shock, hemorrhage
 Dehydration
 Increased protein metabolism
 Increased protein catabolism

67
BUN/Creatinine Ratio

Increased ratio with Tend to be caused by post-

dysproportionate renal conditions that
increased BUN, obstruct urine flow:
slightly increased  Stone
creatinine  Tumor
 Sever infection

68
BUN/Creatinine Ratio

Tend to be caused by - renal

Increased ratio with conditions that decrease kidney
increased BUN, function:
increased creatinine  Acute renal failure
 Chronic renal failure
 Glomerulonephritis
 Tubular necrosis

69
BUN/Creatinine Ratio

Tend to be caused by conditions of

Decreased ratio decreased urea production:
with decreased  Low protein diet
BUN
 Liver disease

70
Uric Acid
• Uric acid is the final breakdown product of purine metabolism

 Purines from the breakdown of ingested nucleic acids or from tissue

destruction, are converted into uric acid, primarily in the liver

• Transported in the plasma from the liver to the kidney

• Readily filtered by glomerulus, but then is handled by tubules

 Reabsorption of 98-100% occurs in the proximal tubules

Not a good test for GFR

 Uric Acid Clinical Significance
• Increased uric acid (Hyperuricemia)
 Gout
 Increased breakdown of nucleic acids: chemotherapy
 Renal disease
• Decreased uric acid (Hypouricemia)
 Severe liver disease
 Fanconi’s syndrome
 Over treatment with ‘allopurinol’
 Disease Correlations
Gout
• a disease found primarily in men

• Diagnosed between 30 and 50 years of age

• Patients have pain and inflammation of the joints caused by

precipitation of sodium urates

• In 25-30% of these patients, hyperuricemia is a result of over

production of uric acid

 Hyperuricemia may be aggravated by a purine rich diet, drugs, or alcohol

• Plasma uric acid concentration is usually greater than 6.0 mg/dL

Disease Correlations

Clinical Chemistry 74
Disease Correlations

• Patients are highly susceptible to the development of renal calculi

• In severe cases, deposits of urates called tophi form in tissue,

causing deformities

• Chronic renal disease causes elevated uric acid concentration

because filtration and secretion are impaired

Clinical Chemistry 75
Determination of uric acid
Folin denis method
• It is based on the oxidation of uric acid in a protein-free filtrate
with subsequent reduction of phosphotungstic acid to tungsten
blue
• Sodium carbonate provides the alkaline pH necessary for color
development
• Tungsten blue absorbs at  = 650-700 nm
• The method lacks specificity
O
Phosphotungstic acid Tungsten blue O H
N N
HN H2N
O- O

O N N O2 H2O2 N
H O N H
H H
Uric Acid Allantoin
Clinical Chemistry 76
 Uricase-peroxidase method
• Principle:

 Uric acid uricase allantion + CO2 + H2O2

 2H2O2 + 4-aminoantipyrine + dichlorophenol sulfonate

peroxidase

quinonamine + 4H2O
( rose in color )
• Uricase-peroxidase method is more specific

 Absorbance of uric acid at   585 nm is monitored

Uricase UV. continuous monitoring

• More specific

• The enzyme that catalyzes the oxidation of uric acid to

allantoin and CO2

• The simplest of methods measures the differential absorption

of uric acid and allantoin at 293 nm

• The difference in absorbance before and after incubation with

uricase is proportional to the uric acid concentration
Specimen collection/handling Uric Acid
• Serum or heparinized plasma
 Process immediately

 Avoid hemolysis; avoid lipemia

 Salicylates: falsely increased results

 Stability: 3-5 days at refrig temps

• Urine
 Timed collection preferred

 Store in refrig during and after collection to inhibit bacterial growth

 Stability: 3-5 days at refrig temps

Uric Acid
Reference Range:
Adult male: 3.5-7.2 mg/dl
 Serum Adult female: 2.6-6.0 mg/dl
Child: 2.2-5.5 mg/dl

 Urine 250-750 mg/24 hr

Renal Clearance Test: Monitor GFR
• Renal clearance expresses volume of blood cleared of a
substance per unit of time
– Example: mL of substance per minute
• Effective and sensitive way of measuring the actual excretory
capacity of the kidney
• Clinically more useful than the other renal function tests
• The concentration of the substance excreted in the urine is
measured and compared to the concentration of the same
substance in the plasma

81
Renal clearance
• The amount of substance cleared by the kidney is generally
expressed as a volume of plasma

• Standard clearance formula:

ml plasma cleared = Uc V
Pc

• U = urine substance concentration (mg/dl)

• V = total volume of urine collected

• P = plasma substance concentration (mg/dl)

82
 Renal clearance
• The clearance rate is roughly proportional to the size of the
kidney and the patient’s body surface
• Clearance corrected for body surface area:
ml plasma cleared/minute= UcV x 1.73
Pc A
• A = body surface area (BSA)

Body surface can be calculated from the weight and the height of
patients using the following equation
log A = (0.425log W) + (0.725log H ) – 2.144

83
Clearance Test: Monitor GFR
• Substance used to monitor GFR must meet the following
criteria:

 Filtered exclusively by glomerulus

 Not reabsorbed by kidney tubules

 Not secreted by kidney tubules

• Most often used = creatinine clearance

Creatinine Clearance (CrCl)
• Why is creatinine clearance most often used to monitor GFR?

 Creatinine freely filtered by glomerulus

 Creatinine not ‘rehandled’ by tubules

 Creatinine is an endogenous substance

 Amount of creatinine produced per day is constant

 Amount of creatinine produced is proportional to muscle

mass
Clearance Test

• Patient preparation

 Patient should be well hydrated

 Avoid coffee and tea (caffeine) on day of test

Creatinine Clearance (CrCL)
Specimen collection/handling

• Timed urine collection: 24 hour preferred

 Measure total volume of urine collected

 Measure urine creatinine (mg/dl)

• Serum/heparinised plasma

 Collect blood specimen sometime during the urine collection

period

 Measure serum/plasma creatinine (mg/dl)

Creatinine Clearance (CrCL)…..
• Standard clearance formula:
UV U = urine creatinine (mg/dl)
P V = total volume of urine collected: ml/min
P = plasma creatinine (mg/dl)
• Clearance corrected for body surface area:
UV x 1.73 A = body surface area (BSA)
P A 1.73 = average BSA
Log A=(0.425log W ) + (0.725log H ) - 2.144
Creatinine Clearance
• Example: Consider the following data:
 Serum creatinine: 1.8 mg/dl
 Urine creatinine: 63 mg/dl
 Total urine volume:1680 ml/24hr = 1680 ml/1440 minutes
 Patient height: 178 cm
 Patient weight: 82 Kg

1. Calculate the urine creatinine (mg/24hr)

2. Calculate the CrCl

3. Calculate the corrected CrCl for body surface area

CrCl vs Urine Creatinine
Consider the following data:
Serum creatinine: 1.8 mg/dl
Urine creatinine: 63 mg/dl
Total urine volume: 1680 ml/24hr = 1680 ml/1440 minutes
1. Calculate the urine creatinine (mg/24hr)

63 mg x 1680 ml x 1 dl = 1058.5 = 1058 mg

dl 24 hr 100 ml 24 hrs
CrCl vs Urine Creatinine
Consider the following data:

Serum creatinine: 1.8 mg/dl

Urine creatinine: 63 mg/dl

Total urine volume: 1680 ml/24hr = 1680 ml/1440 minutes

2. Calculate the CrCl = UV/P

63 mg/dl x 1680 ml = 40.8 = 41 ml

1.8 mg/dl 1440 min min
 CrCl vs Urine Creatinine
Consider the following data:
Serum creatinine: 1.8 mg/dl
Urine creatinine: 63 mg/dl
Total urine volume: 1680 ml/24hr = 1680 ml/1440 minutes
Patient height: 178 cm
Patient weight: 82 Kg
Surface area = 2.00 m2
3. Calculate the corrected CrCl for body surface area

41 ml x 1.73 = 41 x 0.87 = 35.7 = 36 ml

min 2.00 min
Inulin Clearance
• Inulin, a polymer of fructose, is an extremely stable substance
that is not reabsorbed or secreted by the tubules.

• It is not a normal body constituent, however, and must be

infused at a constant rate throughout the testing period.

• A test that requires an infused substance is termed an

exogenous procedure and is seldom the method of choice
Inulin Clearance
• The reference method and most accurate clearance test is the
inulin clearance test however, it is only used as a research
method.

• This makes it potentially riskier and certainly more expensive

than a creatinine clearance test
Estimated GFR (EGFR)
• National Kidney Foundation recommends an EGFR be
calculated each time a serum creatinine is reported

• to detect chronic renal disease earlier

• Predicts GFR based on patient age, sex, body size, race, serum
creatinine

• Do not need to collect timed urine: better for patient

EGFR (ml/min) =
(140 - age) x (Weight in kg) x (0.85 if female)
72 x Serum creatinine in mg/dl
Creatinine Clearance
• Reference Range

Adult male: 97-137 ml/min

Adult female: 88-128 ml/min

EGFR: >59 ml/min

Creatinine Clearance
• Clinical Significance
 Used to monitor GFR
 As renal function fails, CrCl decreases
 Dialysis indicated when CrCl critically low
 (GFR ~ 10-20 mL/min)
• Correlates with increased BUN/ Creatinine ratio with
increased BUN, increased plasma creatinine and decreased
urine creatinine
Quality Control
• A normal & abnormal quality control sample should be
analyzed along with patient samples, using Westgard or other
quality control rules for acceptance or rejection of the
analytical run.

– Assayed known samples

– Commercially manufactured

• Validate patient results

• Detects analytical errors

 Documentation of Renal Function Test Results
• Record patient results in result logbook
• Record QC results in QC logbook
• Retain records for recommended time
Summary
• Renal function tests: metabolic pathways,
methods of analysis, calculations,
interpretations and correlation of results
– BUN/ urea
– Creatinine
– Uric Acid
– Creatinine Clearance
 Reference
1. Burtis, Carl A., and Ashwood, Edward R.. Tietz:
Fundamentals of Clinical Chemistry.
Philadelphia, 2001.
2. Arneson, W and J Brickell: Clinical Chemistry:
A Laboratory Perspective 1st ed. 2007 FA
Davis
3. Burtis, Carl A., and Ashwood, Edward R.. Tietz:
textbook of Clinical Chemistry. Philadelphia,
1999.
QUIZ (5%)
1. The final breakdown product of purine metabolism is
A. Creatinine
B. Urea
C. Uric acid
D. Amino acid
2. Decreased serum creatinine level is mainly due to
A. Congestive heart failure
B. Muscle atrophy
C. Chronic nephritis
D. Muscle diseases such as gigantism

4. what are the conditions that must be met by the substance to be

used to monitor GFR (1.5pts.)

3. What are the causes of decreased plasma level of uric acid?