Glucose Tolerance Test

A glucose tolerance test (GTT) may be used to help diagnose prediabetes and diabetes. In some cases, a GTT may be used as a screening test, but most commonly, a fasting blood glucose (FBG) or hemoglobin A1c test is used for diabetes screening.

One common protocol is the 2-hour glucose tolerance test (GTT). For this test, you may have a fasting glucose test done, then you drink a 75-gram glucose drink (or 1.75 grams per kilogram of body weight if you are a child). Another blood sample is drawn 2 hours after you begin to drink the glucose drink. This protocol “challenges” your body to produce insulin to prevent hyperglycemia while returning the glucose level back to normal.

The GTT is more complicated to perform than other tests used to detect diabetes, such as a FBG or hemoglobin A1c, but is more sensitive and can detect diabetes earlier. Thus, the GTT may sometimes be used to screen for diabetes in certain patient populations (e.g., candidates for kidney transplants).

If the test is used for screening and the initial glucose screening result is abnormal (e.g., hyperglycemia is present at some time during the test), the test may be repeated. The second test result must also be abnormal (e.g., hyperglycemia is present at some time during the test) to confirm a diagnosis of diabetes. Alternatively, an abnormal screening result may be followed by more definitive testing. During the GTT, if only the fasting glucose is in the hyperglycemic range (which is unusual), the repeat test need only be a fasting glucose test.

A GTT may be ordered when you have had abnormal or borderline results on a fasting blood glucose or random glucose test. It may be ordered when your healthcare practitioner wants to use a sensitive test to screen for diabetes.

Normally, the 75-gram glucose drink raises your blood glucose level, which stimulates the pancreas to release insulin into the blood. Insulin allows the glucose to be taken up by cells. As time passes, the blood glucose level is expected to decrease again back to normal. When you are unable to produce enough insulin, or if the body’s cells are resistant to its effects (insulin resistance), then less glucose is transported from the blood into cells and the blood glucose level remains high.

The following table summarizes the meaning of GTT results.

2-Hour Oral Glucose Tolerance Test (OGTT)—non pregnancy

Note: The most severe elevation in glucose is used to characterize your status: normal (e.g., nondiabetic), prediabetes (impaired fasting glucose and/or impaired glucose tolerance) is present, or diabetes (hyperglycemia) is present. If you have symptoms of diabetes, hyperglycemia that is either fasting or at 2 hours will establish the diagnosis of diabetes. If you are asymptomatic, diabetes is diagnosed when any 2 abnormal results are present on the same day or 2 abnormal results are present on 2 different days.

The glucose solution typically tastes similar to a very sweet soda. Substitutes for the glucose drink are not recommended.

While you should fast—nothing to eat or drink for 8 to 14 hours before your test—small sips of water are usually permissible.

Hemoglobin A1c is an indicator of long-term glucose control that is commonly ordered in patients with prediabetes and diabetes and can also be used as a screening test for diabetes. A fasting blood glucose is another test that may be used for diabetes screening and diagnosis. Other less common tests, such as islet autoantibodies, insulin, and C-peptide, may sometimes be performed to help determine the cause of abnormal glucose levels, to distinguish between type 1 and type 2 diabetes, and to evaluate insulin production. Islet autoantibodies, insulin, and C-peptide are not measured to diagnose diabetes.

For type 2 diabetes, which is the most common type of diabetes, losing excess weight, eating a healthy diet that is high in fiber and restricted in carbohydrates, and getting regular amounts of exercise may be enough to lower your blood glucose levels. In many cases, however, medications may be necessary to achieve the desired glucose level. With type 1 diabetes (and with type 2 diabetes that does not respond well enough to oral medications), insulin injections several times a day are necessary. Insulin can also be given by a pump continuously subcutaneously. See the article on Diabetes for more on treatment.

Some medications can increase or decrease your blood glucose level. Be sure to tell your healthcare practitioner about any prescribed or over-the-counter drugs or supplements you take.

Extreme stress can cause a temporary rise in blood glucose. This can be a result of, for example, trauma, surgery, heart attack or stroke.