The glucose tolerance test measures the body's ability to metabolize glucose: Patients with untreated diabetes have high blood glucose levels. Glucose tolerance tests are one of the tools used to make the diagnosis of diabetes.
The oral glucose tolerance test is used to screen pregnant women for gestational diabetes at between 24 and 28 weeks of pregnancy. It may also be used to diagnose diabetes in cases where the disease is suspected despite a normal fasting blood glucose.
The most common glucose tolerance test is the oral glucose tolerance test (OGTT). For adults, the test requires that the patient fast overnight, and then consume 100gm of glucose in water. Blood sugar measurements are then made at 1⁄2, 1, 2, 3, 4, 5 and 6 hour intervals after ingestion of the glucose load. Insulin levels may also be measured.
For infants and children the preparation depends on various factors such as the child's age.