How the Test is Performed

The most common glucose tolerance test is the oral glucose tolerance test (OGTT). You cannot eat or drink anything for at least 8 hours before the test. For the test, you will be asked to drink a liquid containing a certain amount of glucose (usualy 75 grams). Your blood will be taken before you do this, and again every 30 to 60 minutes after you drink the solution. The test takes up to 3 hours.

The intravenous glucose tolerance test (IGTT) is rarely used, and is never used to diagnose diabetes. In this test, glucose is injected into your vein for 3 minutes. Blood insulin levels are measured before the injection, and again at 1 and 3 minutes after the injection. However, the timing may vary.

How to Prepare for the Test

Make sure you eat normally for several days before the test.

Do not eat or drink anything for 8 hours before the test. You cannot eat during the test.

Ask your health care provider if any of the medicines you take can affect the test results.

How the Test Will Feel

Some people feel nauseated, sweaty, light-headed, or faint after drinking the glucose for the test. However, this is uncommon.

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the Test is Performed

Glucose is the sugar the body uses for energy. Patients with untreated diabetes have high blood glucose levels. Glucose tolerance tests are one of the tools used to diagnose diabetes.

Above-normal blood glucose levels can be used to diagnose type 2 diabetes or high blood glucose during pregnancy (gestational diabetes). Insulin levels may also be measured. (Insulin is the hormone produced by the pancreas that moves glucose from the blood into cells.)

The oral glucose tolerance test is used to screen pregnant women for gestational diabetes between 24 and 28 weeks of pregnancy. It may also be used when the disease is suspected, even though the fasting blood glucose level is normal.

Normal Results

Normal blood values for a 75-gram oral glucose tolerance test used to check for type 2 diabetes:

• Fasting: 60 -100 mg/dL
• 1 hour: less than 200 mg/dL
• 2 hours: less than 140 mg/dL. Between 140 - 200 mg/dL is considered impaired glucose tolerance (sometimes called "prediabetes"). It means you are at increased risk for developing diabetes. A glucose level of 200 mg/dL or higher is a sign of diabetes.

Normal blood values for a 50-gram oral glucose tolerance test used to screen for gestational diabetes:

• 1 hour: equal to or less than 140 mg/dL

Normal blood values for a 100-gram oral glucose tolerance test used to screen for gestational diabetes:

• Fasting: less than 95 mg/dL
• 1 hour: less than 180 mg/dL
• 2 hour: less than 155 mg/dL
• 3 hour: less than 140 mg/dL

Note: mg/dL = milligrams per deciliter

The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.

What Abnormal Results Mean

Higher-than-normal levels of glucose may mean you have prediabetes, diabetes, or gestational diabetes.

However, high glucose levels may be related to another medical problem (for example, Cushing syndrome).

Risks

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Considerations

Factors that may affect the test results:

• Acute stress (for example, from surgery or an infection)
• Vigorous exercise

Several drugs may cause glucose intolerance, including:

• Atypical antipsychotic medications, especially olanzapine, quetiapine, and risperidone
• Beta-blockers (for example, propranolol)
• Corticosteroids (for example, prednisone)
• Estrogens
• Oral contraceptives (birth control pills)
• Thiazide diuretics (for example, hydrochlorothiazide)

Before having the test, let your health care provider know if you are taking any of these medications.

Alternative Names

Oral glucose tolerance test

References

American Diabetes Association. Standards of medical care in diabetes -- 2011. Diabetes Care. 2011;33:S11-S61.

Inzucchi SE, Sherwin RS. Type 2 diabetes mellitus. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 248.

Update Date: 5/22/2012

Reviewed by: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.