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What is Glucose Tolerance Test?

The glucose tolerance test determines how your body breaks down sugar.

Glucose tolerance tests are also used to diagnose diabetes. The OGTT is used to screen for, or diagnose diabetes in people with a fasting blood glucose level that is high, but is not high enough (above 125 mg/dL) to meet the diagnosis for diabetes.

How Is The Test Performed?

The most common glucose tolerance test is the oral glucose tolerance test (OGTT).

  • Before the test begins, a sample of blood is taken.
  • The individual is asked to drink a liquid containing a 75 grams amount of glucose. Blood is taken again every 30 to 60 minutes after drinking the solution.

How to Prepare For the Test?

  • Fasting of at least 8 hours before the test. You cannot eat during the test.
  • Ask your health care provider if any of the medicines you take can affect the test results.

How Does The Test Feel?

  1. Drinking the glucose solution is similar to drinking very sweet soda.
  2. Side effects from this test are very uncommon. Some may get nauseated due to glucose intake.
  3. Only a needle prick pain is felt during blood withdrawal.Expand Section

Normal Values

Normal glucose tolerance test values (75g of Glucose)

Fasting: Less than or equal to 100 milligrams per deciliter (mg/dL) or 5.6 millimoles per liter (mmol/L)

1-hour: Less than 184 mg/dL or less than 10.2 mmol/L

2-hour: Less than 140 mg/dL or less than 7.7 mmol/L

You have pre diabetes if the results of your oral glucose tolerance test are:

140 to 199 mg/dL (2 hours after the beginning of the test).

*The ranges may vary from each as well as population group.


High glucose levels may be caused by:

  • Diabetes
  • Gestational diabetes. Targets for OGTT are different during pregnancy.
  • Some medicines, such as corticosteroids, niacin, phenytoin, diuretics, and some medicines used to treat high blood pressure, HIV.
  • Large amounts of the hormone cortisol in the blood (Cushing’s syndrome).

Low glucose levels may be caused by:

  • Certain medicines, to treat diabetes, blood pressure or depression.
  • Decreased production of the hormones cortisol and aldosterone.
  • Problems with the thyroid gland or an underactive pituitary gland.
  • A tumor or other problems of the pancreas.
  • Liver disease.

Many conditions can change blood glucose levels. Your doctor will discuss any significant abnormal results with you in relation to your symptoms and past health.


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