This test is used to establish the presence of diabetes mellitus. The test should be performed on patients who have been eating an unrestricted diet containing at least 300 grams of carbohydrate/day. Patients should also have been physically active for 3 days prior to testing, and must fast for 10 hours immediately prior to the test. Once patients have been prepared as above, a baseline blood sugar value is obtained. Next, a 75 gram glucose load is administered and then blood sugar values are drawn at 30, 60, 90, and 120 minutes.

Interpretation: Diabetes mellitus is diagnosed when either:

1) fasting serum glucose values are greater than 126 mg/dL on two separate occasions or,

2) a 2-hour glucose value greater than 200 mg/dL is present and at least one other value is greater than 200 mg/dL during the test (two values greater than 200 mg/dL are needed to establish the diagnosis) .

The diagnosis of impaired glucose tolerance (IGT) is made if during the GGT there is a 2-hour value between 140 and 199 mg/dL. Patients with IGT are thought to have a propensity to progress to diabetes mellitus, but in actuality only 25% of these patients will progress. When acromegaly is suspected, the glucose tolerance test is performed with growth hormone levels drawn at the time of the glucose values. Acromegaly is confirmed if the growth hormone levels fail to suppress to less than 1 ng/mL.