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Glucose tolerance test

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What is a Glucose tolerance test?

A glucose tolerance test measures the rate at which glucose is cleared from the bloodstream. This is a diagnostic test used to detect diabetes, pre diabetes as well as gestational diabetes. The oral glucose tolerance test or the OGTT as it is otherwise known, reveals important information regarding carbohydrate metabolism within your body. Though it is quite normal for glucose levels to rise after a meal, it should return to normal levels as the body releases insulin and uses it up. If blood glucose continues to remain high, this indicates the presence of diabetes.

Why is it conducted?

The presence of diabetes causes long term damage to internal organs. Timely detection helps to keep the effects of this disease within control.

  • The most common reason for conducting the glucose tolerance test is to detect diabetes, especially type II.
  • It can help to pinpoint insulin resistance in which cells do not respond adequately to insulin.
  • It is a more accurate test than the finger prick test for diabetes.

Who should go for it?

  • Persons who are suspected to have diabetes or exhibit symptoms that indicate diabetes.
  • Persons whose random glucose test or fasting blood sugar test is elevated.
  • Persons whose blood sugar levels are borderline.
  • Women who are pregnant are advised to undergo this test to rule out gestational diabetes, as it can have serious repercussions for the baby.


This is a blood test in which you are required not to eat or drink at least from 8 hours before the test is conducted. However, you should be having your normal diet before that. A sample of blood is taken in the beginning to get your fasting blood sugar or the baseline blood glucose level before you drink glucose. After that you are given a drink that contains 75 grams of glucose. Normally, another blood sample is taken two hours after you have had the glucose drink. In some cases, samples of blood are taken every half an hour up till 2 hours to gauge the rate of glucose clearance from the blood.

You cannot eat anything for the duration of the test as it can affect the results, though you may drink water if you wish.

Interpretation of results

The normal ranges are as follows:

  • For the fasting or baseline test before the glucose drink: 60 -100 mg/dL
  • For the blood test 1 hour after the glucose drink: should be lower than 200 mg/dL
  • For the blood test 2 hours after the glucose drink: should be lower than 140 mg/dL

Results after 2 hours that are higher than these, that is, within 140 mg/dL to 200 mg/dL point towards the prediabetes stage. This is also known as impaired glucose tolerance. At this stage, lifestyle changes can help to delay the onset of full blown diabetes. However, results above 200 mg/dL are a definite indication of diabetes.

Certain factors can affect the results of the test and should be taken into consideration. Some medicines that contribute to higher blood glucose levels are corticosteroids, betablockers (propranolol), medicines used for the treatment of high blood pressure or HIV.

If you are on prolonged bed rest, then again this test will not produce reliable results. Besides, do not have this test if you are suffering from illness of any kind, or are under stress.

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Written by: Nandita tripati

Date last updated: January 10, 2015

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