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.

How to Prepare for the Test

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)

How the Test will Feel

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.

Why the Test is Performed

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.

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