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 the person has not had anything to eat or drink during the night, a blood sample is taken on the morning of the test. This is the fasting blood glucose value. It provides a baseline for comparing other glucose values. The person being tested drinks a special sugar solution. A blood sugar sample is taken from a vein in the person's arm at timed intervals 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 may develop during pregnancy.