Oral Glucose Tolerance Test
OGGT
CLUCOSE TOLERANCE TEST (GTT) AND ITS
INTERPRETATION RELATED TO DIABETES MELLTUS
• The blood glucose levels must be maintained within the limits of
65-110 mg/dl under hormonal control.
• Levels above the normal range are called hyperglycemic and those
below are called hypoglycemic.
Factors affecting this delicate balance
1- Storage - as glycogen (glycogenesis vs glycogenolysis).
2- Supply - as sugar in diet and from gluconeogenesis.
3- Demand - use in metabolism.
4- Conversion - into fat or other carbohydrates.
• Following CHO food intake, the blood glucose level rise temporary to 160-180
mg/dl and then return to normal fasting level within 2-3 hours.
• This effect of ingested carbohydrates can be studied under standard
conditions by means of glucose tolerance test (GTT)
• GTT is considerable use in investigating abnormalities of carbohydrate metabolism and
causes, leading to glucosuria (presence of glucose in urine).
• Two opposing forces are responsible for the time course;
✓Some elderly individuals with chronic kidney disease (chronic nephritis), there may not be
glucosuria despite hyperglycemia (250-300 mg /dl glucose levels) as in the renal threshold
has risen higher.
✓ While other individuals, in whom the renal threshold has become low (found generally in
pregnant ladies in third trimester as a results from
indicative glomerular filtration rate and decreased tubular reabsorption of glucose, It is
obviously self limiting and requires no treatment), may show glucosuria, through they are not
hyperglycemia. Therefore, presence of glucosuria is not always indicative of hyperglycemia or
diabetes mellitus.
• Conversely, a negative test glucosuria does not exclude hyperglycemia (diabetes mellitus),
hence there is need to investigate such individuals by GTT
GTT is indicated in following situations:-
1. To investigate and confirmed glucosuria of any degree.
2. To evaluate obese person particularly with a family history of diabetes.
3. To establish presence or absence of diabetes mellitus in patient in
whom the fasting or postprandial blood glucose is normal or equivocal
(that is not desire).
4. In clinical disorders suggestive of diabetes but where fasting level of
blood glucose is found normal.
5. During pregnancy with clinical pointers of glucosuria, abortion or still
birth, family history of diabetes and excessive weight gain especially
with pre-existing obesity.
1. Normal response.
a. A normal curve (Curve A)
Should have the following:
1. The fasting blood sugar is within normal limits of 70-110mg/dl
(measured by glucose oxidase method).
2. The highest value for blood sugar does not exceed the renal
threshold i.e.160-180 mg/dl.
3. The highest value for blood sugar is reached within the first hour.
4. The fasting level is reached again within 2 1/2 hrs.
5. There is no sugar or ketone bodies in any specimen of urine
b. Lag Curve (Curve B):
1. The fasting blood sugar is normal but it rises rapidly in the first half
to one hour and exceeds the renal threshold, so the corresponding
urine specimen contains sugar.
2. The return to normal is rapid and complete.
3. This type of curve is obtained in following states:-
• Hyperthyroidism (due to rapid glucose absorption).
• After gastro-enterectomy or gastrectomy, when there is rapid entry of glucose in the ileum leading to rapid
absorption.
• During Pregnancy.
• In severe liver disease; probably due to a diminished glycogenesis.
• In early diabetes.
4. A Patient showing a lag curve should be reviewed after about six months or after a period of a month on a
carbohydrate diet
Abnormal responses.
a. Decrease Glucose Tolerance
1. Diabetic curves: Mild diabetes (Curve C).
2. Diabetic curves: Severe diabetes (Curve D).
• These diabetic types of curve show one or more of the followings:
• The fasting blood sugar is definitely raised, 120mg/dl or more, in
majority of cases.
• The highest value is usually reached after one to one and half hours.
• The highest value exceeds the normal renal threshold. The urine contains
sugar, except in some chronic diabetics or nephritis that have raised renal
threshold for glucose.
• The blood sugar does not return to the fasting level within 2 1/2 hrs. This is
the most characteristic feature of true diabetes mellitus
b. Increase Glucose Tolerance
1. Islet-cell tumor and insulinoma (curve E).
• The fasting or base level of glucose is below normal, peak is also
low, often has a plateau.
• There is no glucosuria.
• The glucose concentration falls to hypoglycemia levels by third or fourth hours
and may be altered with severe symptoms of sweating, convulsion and coma
(may be mistaken for epilepsy or neurosis).
2. Reactive hypoglycemia (Curve F):
• In this state the alimentary hyperglycemia, with rapid absorption of
glucose from gut leads to rapid rise within 1/2hrs.
• The insulin response is normal but it cannot prevent moderate hyperglycemia and
responds by excess secretions which carry blood glucose to hypoglycemic range within
2hrs and the curve dips down the fasting blood glucose levels, which remain within
normal limits.
• It is seen in conditions of rapid absorption of glucose as in lag curve (curve B), in
neurotic individuals and in some incipient diabetics in whom the response of insulin
output is delayed.
3. Flat Curve (Curve G):
• In this state, blood glucose levels fail to rise after
glucose load and curve assumes a flat plateau.
• This curve indicates malabsorption. It can also occur
in Addison's disease due to lack of glucocorticoids and
in hypopitutarism due to lack of growth hormone.
Diabetes can be classified into the following general categories:
1.Type 1 diabetes (due to autoimmune ß-cell destruction, usually leading to
absolute insulin deficiency, including latent autoimmune diabetes of
adulthood)
2.Type 2 diabetes (due to a progressive loss of ß-cell insulin secretion
frequently on the background of insulin resistance)
3.Specific types of diabetes due to other causes, e.g., monogenic diabetes
syndromes (such as neonatal diabetes and maturity-onset diabetes of the
young), diseases of the exocrine pancreas (such as cystic fibrosis and
pancreatitis), and drug- or chemical-induced diabetes (such as with
glucocorticoid use, in the treatment of HIV/AIDS)
4.Gestational diabetes mellitus (diabetes diagnosed in the second or third
trimester of pregnancy that was not clearly overt diabetes prior to gestation
This test can be used to check for:
▪ Type 2 Diabetes
▪ Gestational Diabetes
▪ Obesity Disease
▪ Hypertension
▪ Metabolic syndrome
▪ Vascular disease
DISCUSSION The patient was shown to have type 1 diabetes
mellitus and had presented in diabetic ketoacidosis, with
hyperglycaemia, hyponatraemia, hyperkalaemia and a
metabolic acidosis.
A 24-year-old woman presented to the casualty department in a coma. The
relevant biochemical results were as follows:
Plasma Sodium 130 mmol/L (135–145)
Potassium 5.9 mmol/L (3.5–5.0)
Bicarbonate 10 mmol/L (24–32)
Chloride 92 mmol/L (95–105)
Glucose 35 mmol/L (5.5–11.1)
pH 7.10 (7.35–7.45)
Urine was positive for ketones.
Which of the following is the most likely diagnosis?
Explain briefly this carve A
Explain briefly this carve B
• What is the different between normal and flat carve in OGTT?
• At which condition do you expect curve B ?
• Define the abnormal response of OGTT?
Case history 25
A 52-year old man attends his general practice for a blood test as part of
cardiovascular risk assessment. He has been asked to attend fasting. Blood glucose is
found to be 7.1 mmol/L.1-What is diagnosis ?
Case history 26
A 22-year-old patient with diabetes comes to the Accident and Emergency
department. She gives a 2-day history of vomiting and abdominal pain. She is drowsy
and her breathing is deep and rapid. There is a distinctive smell from her breath. What
is the most likely diagnosis? Which bedside tests could you do to help you to confirm
this diagnosis? Which laboratory tests would you request?
Case history 27
A 25-year-old woman with IDDM complained of repeated episodes of sleep
disturbances, night sweats and vivid, unpleasant dreams. What is the most likely
cause of this woman’s symptoms and how might the diagnosis be confirmed?
Case history 25 Repeat the fasting blood glucose. The diagnosis of diabetes mellitus is not confirmed
until specimens collected on at least two separate occasions place the patient in the diabetic
category. Case history 26 By far the most likely diagnosis in this case is diabetic ketoacidosis. This
may be precipitated by a number of conditions, such as infection. This may have caused anorexia
and, thus, the patient may have omitted to take her insulin. Trauma can increase a patient’s
requirement for insulin but there is nothing to suggest that in this case. The blood glucose can be
checked at the bedside as can a specimen of urine for the presence of ketones. The laboratory tests
that may be requested are urea and electrolytes to assess renal function, the presence or absence
of hyperkalaemia and the serum sodium concentration. The patient’s acid–base status should be
assessed to quantitate the severity of the acidosis present, and the blood glucose should be
accurately measured. These results will influence the patient’s treatment. It is essential in cases such
as this that samples of blood and urine and, if appropriate, sputum are sent to the microbiological
laboratory to look for the presence of infection. Case history 27 Nocturnal hypoglycaemia is the
most likely cause of this woman’s symptoms. The diagnosis can be made by measuring her blood
glucose while she is symptomatic. However, this can be distressing to patients and is not always
feasible. Indirect evidence of nocturnal hypoglycaemia may be obtained by measuring her urinary
catecholamine excretion or urinary cortisol excretion overnight. A further clue may be obtained if
the 82 Case history comments 167 woman’s glycated haemoglobin level indicates good diabetic
control in the face of hyperglycaemia during the day. In many such cases a diagnosis of nocturnal
hypoglycaemia is inferred if the symptoms are relieved by changing the insulin regimen or getting
the patient to eat more food before she retires at night.
