The glucose tolerance test measures the body's ability to metabolize glucose.

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 make the diagnosis of diabetes.

The most common glucose tolerance test is the oral glucose tolerance test (OGTT). After an overnight fast, a patient drinks a solution containing a known amount of glucose. Blood is obtained before the patient drinks the glucose solution, and blood is drawn again every 30 to 60 minutes after the glucose is consumed for 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, patients are given a known amount of glucose by vein for 3 minutes, and blood insulin levels are measured before glucose and at 1 and 3 minutes. Insulin levels below a standard threshold may predict the development of type 1 diabetes in some patients.

Fast for 12 hours and do not eat during the test. Consult your health care provider if you are using medication 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 diagnose diabetes in research studies and in cases where the disease is suspected despite a normal fasting blood glucose.

For a 75-gram oral glucose tolerance test used to check for type 2 diabetes, normal (nondiabetic) blood values are:

• fasting: 60 to 110 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

For a 50-gram oral glucose tolerance test used to screen for gestational diabetes, normal blood values at 1 hour are less than 140 mg/dL.

For a 100-gram oral glucose tolerance test used to screen for gestational diabetes, normal blood values are:

• 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 indicate

• Diabetes
• Gestational diabetes

High glucose levels may be related to another clinical problem (for example, Cushing's 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 the following:

• Thiazide diuretics (e.g., hydrochlorothiazide)
• Beta-blockers (e.g., propranolol)
• Oral contraceptives
• Corticosteroids (e.g., prednisone)
• Some psychiatric medications

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