The glucose tolerance test is a laboratory method to check how the body breaks down (metabolizes) sugar.
Oral glucose tolerance test
How the test is performed
The most common glucose tolerance test is the oral glucose tolerance test (OGTT). You cannot eat or drink anything after midnight before the test. For the test, you will be asked to drink a liquid containing a certain amount of glucose. Your blood will be taken before you do this, and again every 30 to 60 minutes after you drink the solution. The test takes up to 3 hours.
The intravenous glucose tolerance test (IGTT) is rarely used. In this test, glucose is injected into your vein for 3 minutes. Blood insulin levels are measured before the injection, and again at 1 and 3 minutes after the injection, although the timing may vary.
How to prepare for the test
Make sure you eat normally for several days before the test.
Do not eat or drink anything for 8-10 hours before the test. You cannot eat during the test.
Ask your health care provider if you are using medications that can interfere with the test results.
How the test will feel
Some people feel nauseated or sweaty after drinking the glucose for the test.
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed
Glucose is the sugar the body uses for energy. Patients with untreated
Above-normal blood glucose levels can be used to diagnose
The oral glucose tolerance test is used to screen pregnant women for gestational diabetes between 24 and 28 weeks of pregnancy. It may also be used in cases where the disease is suspected, despite a normal fasting blood glucose level.
Review Date: 05/23/2010
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.