Glucose is the sugar that the body uses for energy. Patients with diabetes mellitus have high blood glucose levels. Glucose tolerance tests are one of the tools for making 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 and urine are obtained before the patient drinks the glucose solution, and blood is drawn again every hour after the glucose is consumed for up to three hours. Blood glucose levels above normal limits at the times measured can diagnose Type 2 ("non-insulin-dependent") diabetes or gestational diabetes (high blood glucose during pregnancy).

Intravenous glucose tolerance tests (IGTT) are uncommon. Patients are given a known amount of glucose by vein for three minutes, and blood insulin levels are measured before glucose and at one and three minutes. Insulin is the hormone produced by the pancreas that moves glucose from the bloodstream into cells. Insulin levels below a standard threshold are predictive of Type 1 ("insulin-dependent") diabetes in some patients.

Fast for 12 hours and do not eat during the test. Consult the health care provider if you are using medication that can interfere with the test results. (See special considerations.)

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:

• infant test or procedure preparation (birth to 1 year)
• toddler test or procedure preparation (1 to 3 years)
• preschooler test or procedure preparation (3 to 6 years)
• schoolage test or procedure preparation (6 to 12 years)
• adolescent test or procedure preparation (12 to 18 years)

The test involves only normal urination, and there is no discomfort.

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 indicated to screen pregnant women for gestational diabetes mellitus between 24 and 28 weeks of pregnancy. It may also be used to diagnose diabetes mellitus in research studies involving diabetics and in cases where diabetes mellitus is suspected despite a normal fasting blood glucose.

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

• fasting: 60 to 110 mg/dl
• 1 hour: less than 200 mg/dL
• 2 hour: less than 140 mg/dL

For a 50 gram oral glucose tolerance test used to screen for gestational diabetes, normal (non-diabetic) blood values are:

• 1 hour: less than 140 mg/dL

For a 100 gram oral glucose tolerance test used to screen for gestational diabetes, normal (non-diabetic) 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

Normal urine results:

• negative (no glucose present)

Greater-than-normal levels of glucose may indicate

• diabetes
• gestational diabetes

Diabetes may be a manifestation of another clinical problem (for example, Cushing's syndrome).

Risks from venipuncture include:

• excessive bleeding
• fainting or feeling lightheaded
• hematoma (bruise, 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:

• acutestress (for example, from surgery or an infection)
• vigorous exercise

Several drugs may cause glucose intolerance. Some examples include thiazide diuretics (e.g., hydrochlorothiazide), beta-blockers (e.g., propranolol), oral contraceptives, corticosteroids (e.g., prednisone), and some psychiatric medications.