The glucose tolerance test is a laboratory method to check how the body breaks down (metabolizes) blood sugar.

Glucose is the sugar that the body uses for energy. Patients with untreated diabetes have high blood glucose levels. Glucose tolerance tests are one of the tools used to diagnose diabetes.

The most common glucose tolerance test is the oral glucose tolerance test (OGTT). You can not 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.

Blood glucose levels above normal limits at the times measured can be used to diagnose type 2 diabetes or gestational diabetes (high blood glucose during pregnancy). Insulin levels may also be measured. (Insulin is the hormone produced by the pancreas that moves glucose from the bloodstream into cells.)

The intravenous glucose tolerance test (IGTT) is not often 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. This test may predict the development of type 1 diabetes in some patients.

Make sure you eat normally for several days before the test.

Do not eat or drink anything for 12 hours prior to the test. You can not eat during the test.

Ask your health care provider if you are using medications that can interfere with the test results.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.