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 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 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. (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/procedure preparation (birth to 1 year)
• Toddler test/procedure preparation (1 to 3 years)
• Preschooler test/procedure preparation (3 to 6 years)
• Schoolage test/procedure preparation (6 to 12 years)
• Adolescent test/procedure preparation (12 to 18 years)

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. 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

Note: mg/dL = milligrams per deciliter

What abnormal results mean   

What the risks are   

Special considerations   

(From the National Institute of Health)

See also

Blood Glucose, Glucose
 

Note:

This diabetes health education project is supported by Chong's Health Care at http://www.cljhealth.com, one of the leading companies in the discovery of alternative medicines for diabetes.