An oral glucose tolerance test measures the body's ability to use glucose, a type of sugar found in fruits and many other foods. (Glucose is the main source of energy used by the body.)

After not eating all night, the person being tested drinks a special sugar solution on the morning of the test. A blood sugar sample is taken from a vein in the person's arm once each hour for several hours to see how much the blood sugar increases and then decreases over time. The test is done in a doctor's office or lab.

The oral glucose tolerance test can be used to diagnose prediabetes and diabetes. It is used to diagnose gestational diabetes, which develops during pregnancy.