The glucose tolerance test is a laboratory method to check how the body breaks down (metabolizes) blood sugar.

Glucose is the sugar that 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.

The most common glucose tolerance test is the oral glucose tolerance test (OGTT). You can not eat or drink anything after midnight before the test. For the test, you will be asked to drink a liquid containing a certain amount of glucose. 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.

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

The intravenous glucose tolerance test (IGTT) is not often used. 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. This test may predict the development of type 1 diabetes in some patients.

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

Do not eat or drink anything for 12 hours prior to the test. You can not eat during the test.

Ask your health care provider if you are using medications that can interfere with the test results.

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

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 to in cases where the disease is suspected, despite a normal fasting blood glucose.

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

• Fasting: 60 to 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 or pre-diabetes. This group is at increased risk for developing diabetes. Greater than 200 mg/dL is diagnostic of diabetes mellitus

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

• 1 hour: less than 140 mg/dL

Normal blood values for 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

Greater than normal levels of glucose may mean you have diabetes or gestational diabetes.

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

The risks of drawing blood from a vein include:

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

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.

Interfering factors:

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

Several drugs may cause glucose intolerance, including:

• Thiazide diuretics (for example, hydrochlorothiazide)
• Beta-blockers (for example, propranolol)
• Oral contraceptives (birth control pills)
• Corticosteroids (for example, prednisone)
• Certain psychiatric medications

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