Glucose Tolerance Test

24th September 2021

The Oral Glucose tolerance test (OGTT) is a blood test to determine the ability of a person to metabolize a given load of glucose. It is a well-standardized test and is highly useful in diagnosing doubtful cases of diabetes. In conventional OGTT blood and urine sample is taken at the half-hourly interval for the next two and half hours. Six samples are taken including zero hour sample. Glucose is checked in all the blood samples and glucose in urine samples is checked qualitatively. Following the glucose load, the plasma glucose reaches a peak value at 1 hour and returns to normal fasting level by about 2-2.5 hours. In insulin deficiency utilization of glucose is impaired. The test is not needed in proven cases of diabetes.


Indications of OGTT

  • In asymptomatic patients with sustained or transient glycosuria. ( glucose in the urine)
  • In persons with symptoms of diabetes but no glucose in urine and blood glucose level within normal range.
  • A person with a family history of diabetes but no symptoms or positive blood findings.
  • A person with or without symptoms of diabetes who has one abnormal blood finding.
  • In patients with retinopathy and neuropathy of unknown origin.
  • Excessive weight gain in pregnancy with a past history of delivering large babies (more than 4kg) or a past history of miscarriage.

Types of Glucose Tolerance Test

  • Standard or classical oral glucose tolerance test
  • Mini or modern glucose tolerance test
  • Intravenous glucose tolerance test

Standard oral glucose tolerance test

Glucose tolerance curves

Preparation of the patient

  • The patient is instructed to have good carbohydrates diet for three days. A diet containing 30-50 g of carbohydrates is taken in the evening prior to the test.
  • Any medicine (such as glucocorticoids, Diuretics, Phenytoin, niacin, or contraceptives) likely to influence glucose readings is avoided at least 2 days before the test.
  • strenuous exercise should be avoided on the previous day is to be avoided. Smoking should be avoided during the test.
  • The patient should not take food after 8PM and breakfast to ensure 12 hours of fasting.

Conducting the OGTT

  • Fasting blood and urine samples are taken. These are called zero samples.
  • A loading dose of 75 g of glucose dissolved in one cup of water is given. (the dose is fixed in adults irrespective of body weight) In children, 1.75g per kg body weight glucose is given.
  • Blood and urine samples are taken at half hourly intervals for the next two and a half to three hours to test for glucose.

Lab findings in a normal person

Time /minutesFasting 30 60 90 120150180
Blood Glucose(mg/dl)891021451221158570
Urine Glucose nilnilnilnilnilnilnil

In a normal person, a peak value of blood glucose is seen after one hour due to the absorption of glucose from the intestines. Insulin is released and then there is a fall in blood glucose with time due to glucose utilization promoted by insulin. Normal blood glucose is achieved after 150-180 minutes. The blood glucose readings can be plotted on a graph against time. The curve obtained is called the glucose tolerance curve.


Lab findings in a diabetic patient

Blood Glucose (mg/dl)205220300310270248220
Urine Glucose+++++++

n diabetic patient’s fasting blood glucose is higher than normal (60-110 mg /dL) The highest level after one to one and half hours exceed the renal threshold. Glucose is found in almost all urine samples. The blood glucose level does not return to the fasting level even after two and a half hours.

Modern or Mini Glucose Tolerance Test

In the mini glucose tolerance test, only two samples are collected according to WHO recommendations. Fasting sample and 2 hours post glucose load blood sample. Urine samples are also collected at the same time. the result is interpreted according to the following criteria:

NormalImpairedDiabetes Mellitus
Fasting Blood Sugar (mg/dL)<110110-125≥126
2 hours after Glucose<140140-199≥200

Intravenous Glucose Tolerance Test

Intravenous glucose tolerance test is carried out in patients with gastrointestinal problems like celiac disease or enteropathies as glucose load will not be well absorbed. It is carried out by injecting 25 g of glucose dissolved in 100 ml of distilled water within five minutes. Then a blood sample is taken every 10 minutes for one hour. The peak value is seen within a few minutes and the value comes back to near normal within 45-60 minutes. In diabetic patients decline is slow.


Causes for abnormal GTT curve

Impaired Glucose Tolerance Test

In impaired glucose tolerance(IGT) or impaired glucose regulation (IGR), plasma glucose levels are above the normal levels, but below the diabetic levels. These people need careful follow-up as IGT may progress to frank diabetes

Impaired Fasting Glycemia

In impaired fasting glycemia, plasma fasting glucose level is above the normal value, but plasma glucose after 2 hours is within normal limits. These patients also need constant check-ups.

Gestatioal Diabetes Mellitus

Some pregnant women develop glucose intolerance during pregnancy. A glucose challenge test is done at 22-24 weeks of pregnancy by giving 50 g of glucose load regardless of the times. If the 2-hour glucose value is more than 140mg/dl then the test is repeated with a glucose load of 75g to exclude or confirm GDM.

Women with GDM are at increased risk for developing diabetes in the future. GDM is associated with increased neonatal mortality. Increased blood glucose levels in the mother cause the fetus to secrete more insulin, causing stimulation of fetal growth and increased birth weight.

Alimentary Glucosuria

Here the fasting and 2-hour values are normal, but an exaggerated rise in blood glucose is seen following the ingestion of glucose. This is due to an increased rate of absorption of glucose from the intestines. This is seen in patients after gastrectomy and in cases of hyperthyroidism.

Renal Glucosuria

The Normal renal threshold for glucose is 175-180mg/dl. If blood glucose rises above this, glucose starts to appear in the urine. In some patients blood glucose reading and glucose tolerance curve are normal but there is glucose in some samples of urine because there is a lowering of the renal threshold. Causes of renal glycosuria can be:

  • Early diabetes mellitus
  • Pregnancy (renal threshold is lowered physiologically in pregnancy. It is harmless and will not progress to diabetes.
  • Renal disease e.g. Fanconi’s syndrome
  • Heavy metal poisoning
  • Deficiency of a carrier protein (SGLT-2)
  • Children of diabetic parents.

In some patients, a lag curve is seen. In whom Fasting blood glucose is normal. The peak level is seen after one hour which exceeds the renal threshold and glucose appears in the urine. The decline is rapid and a normal fasting glucose level is achieved. This can be seen in pregnancy, Hyperthyroidism, and after gastroenterostomy.

Factors Affecting Glucose Tolerance Test

  • Stress and acute infections( cortisol is secreted)
  • Liver diseases
  • Hyperthyroidism (there is a steep rise in a curve)
  • Hypothyroidism. A flat curve is observed due to decreased glucose absorption from the gut.
  • Starvation. There is a rise in counter-regulatory hormones so glucose tolerance decreases.

Conditions in which Glucose tolerance increases

Glucose tolerance increases in the conditions causing hypoglycemia

  • Hypopituitarism
  • Hyperinsulinism
  • Hypothyroidism
  • Adrenal cortex hypofunction
  • Decrease gastrointestinal absorption for example in sprue, celiac disease.

Conditions in which glucose tolerance decreases

Glucose tolerance decrease in all conditions causing hyperglycemia such as:

  • Diabetes Mellitus
  • Hyperactivity of anterior pituitary gland
  • Cushing’s syndrome
  • Hyperthyroidism
  • Stress

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