Glucose Tolerance Test

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How is the Test Performed?

Glucose is the sugar that the body uses for energy. Patients with diabetes mellitus have high blood glucose levels. Glucose tolerance tests are one of the tools for making the diagnosis of diabetes.

The most common glucose tolerance test is the oral glucose tolerance test (OGTT). After an overnight fast, a patient drinks a solution containing a known amount of glucose. Blood is obtained before the patient drinks the glucose solution, and blood is drawn again every 30 to 60 minutes after the glucose is consumed for up to 3 hours.

Blood glucose levels above normal limits at the times measured can 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, patients are given a known amount of glucose by vein for 3 minutes, and blood insulin levels are measured before glucose and at 1 and 3 minutes. Insulin levels below a standard threshold may predict the development of type 1 diabetes in some patients.