Glucose Tolerance Test

24th September 2021

Oral Glucose tolerance test (OGTT) is a blood test to determine the ability to utilize glucose. In insulin deficiency utilization of glucose is impaired. OGTT is helpful to determine doubtful cases. Initially, fasting blood glucose is taken after that a loading dose of glucose is given and then blood sugar is recorded at regular intervals. Test is not needed in proven cases of diabetes and it should not be performed in acutely sick patients.


Indications of Glucose Tolerance Test

  • In asymptomatic patient with sustained or transient glycosuria. ( glucose in urine)
  • In persons with symptoms of diabetes but no glucose in urine and blood glucose level within normal range.
  • Person with a family history of diabetes but no symptoms or positive blood finding.
  • Person with or without symptoms of diabetes having one abnormal blood finding.
  • In patients with retinopathy and neuropathy of unknown origin.
  • In women who have delivered large babies.

Conditions in which OGTT decreases

OGTT decrease in all conditions causing hyperglycemia such as:

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

Conditions in which OGTT increases

OGTT increases in condition causing hypoglycemia

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

Types of Glucose Tolerance Test

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

Standard oral glucose tolerance test

Glucose tolerance curves

Patient instructed to have normal carbohydrates for three days prior to the test. Any medicine (such as glucocorticoids, Diuretics, Phenytoin,niacin, contraceptives) likely to influence glucose reading and strenuous exercise should be avoided on that day. Smoking should be avoided during the test.

  • Fasting blood and urine sample taken. These are called zero samples.
  • A loading dose of 75 g of glucose given dissolved in one cup of water. In children 1.75g per kg body weight glucose is given.
  • Blood and urine samples are taken half hourly interval for next two and 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 normal person a peak value of blood glucose is seen after one hour due to absorption of glucose from intestines. Insulin is released and then there is fall in blood glucose with time due to glucose utilization promoted by insulin. The normal blood glucose is achieved after 150 minutes. The blood glucose readings can be plotted on a graph against time.The curve obtained is called glucose tolerance curve.


Lab findings in a diabetic patient

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

In diabetic patient 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 the urine samples. The blood glucose level does not return to the fasting level even after two and half hour.

In some patients blood glucose reading are and glucose tolerance curve is normal but there is glucose in some sample of urine because there is lowering of renal threshold. Causes of renal glycosuria can be:

  • Early diabetes mellitus
  • Pregnancy
  • Renal disease
  • 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. Peak level is seen after one hour which exceed renal threshold and glucose appear in urine. Decline is rapid and normal fasting glucose level is achieved . This can be seen in pregnancy, Hyperthyroidism and after gastroenterostomy.


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 blood sample is taken every 10 minutes for one hour. Peak value is seen within few minutes and value comes back to near normal within 45-60 minutes. In diabetic patients decline is slow.


Factors Affecting Glucose Tolerance Test

  • Acute infections( cortisol is secreted)
  • Liver diseases
  • Hyperthyroidism (there is a steep rise in curve)
  • Hypothyroidism. A flat curve observe due to decrease glucose absorption from the gut.
  • Starvation. There is rise in counter regulatory hormones so GTT increases.

Modern or Mini Glucose Tolerance Test

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

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

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