The 3 hour glucose tolerance test is not done routinely to diagnose type 2 diabetes. However the test is considered as the gold standard for making the diagnosis of type 2 diabetes. Presently it is used most often to diagnose gestational diabetes (diabetes in pregnant women).
Before doing an oral glucose tolerance test, you will have to fast overnight (at least 8 but not more than 16 hours). The next morning sample for fasting plasma glucose level is taken. After this you will be given 75 grams of glucose (100 grams for pregnant women) solution to drink.
Then blood samples are taken to measure the blood glucose---In the classic test blood glucose level is measured five times over a period of 3 hours.
Based on the results of 3 hour glucose tolerance test the following diagnoses may be made:
- Normal response: In people with normal glucose tolerance test the 2-hour glucose level remains lower than 140 mg/dl, and all values between 0 and 2 hours remain lower than 200 mg/dl.
- Impaired glucose tolerance: If your fasting plasma glucose is less than 126 mg/dl and the 2-hour glucose level is between 140 and 199 mg/dl you have impaired glucose tolerance test
- Diabetes: If your fasting plasma glucose is greater than 126 mg/dl and/or the 2-hour glucose level is greater than mg/dl repeatedly (done on different days)---you have diabetes.
- Gestational diabetes: A woman is diagnosed with gestational diabetes if she has any two of the following after 100g OGTT---fasting plasma glucose > 95 mg/dl, 1-hour glucose > 180 mg/dl, 2-hour glucose level > 155 mg/dl, or a 3-hour glucose level > 140 mg/dl.
The three hour glucose tolerance test can help in early diagnosis and disease management by permitting early initiation of therapy. This can help to prevent or delay the development of diabetes related complications.
Dr Poonam Sachdeva, our in-house medical expert talks about the why what and how in diabetes.
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