The glucose tolerance test is a laboratory method to check how the body breaks down (metabolizes) sugar.
Oral glucose tolerance test
The most common glucose tolerance test is the oral glucose tolerance test (OGTT). You cannot 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.
The intravenous glucose tolerance test (IGTT) is rarely 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, although the timing may vary.
Make sure you eat normally for several days before the test.
Do not eat or drink anything for 8-10 hours before the test. You cannot eat during the test.
Ask your health care provider if you are using medications that can interfere with the test results.
Some people feel nauseated or sweaty after drinking the glucose for the test.
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Glucose is the sugar 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.
Above-normal blood glucose levels can be used to diagnose type 2 diabetes or high blood glucose during pregnancy (gestational diabetes). Insulin levels may also be measured. (Insulin is the hormone produced by the pancreas that moves glucose from the bloodstream into cells.)
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 in cases where the disease is suspected, despite a normal fasting blood glucose level.
Normal blood values for a 75-gram oral glucose tolerance test used to check for type 2 diabetes:
Normal blood values for a 50-gram oral glucose tolerance test used to screen for gestational diabetes:
Normal blood values for a 100-gram oral glucose tolerance test used to screen for gestational diabetes:
Note: mg/dL = milligrams per deciliter
Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
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).
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.
Other risks associated with having blood drawn are slight but may include:
Factors that may affect the test results:
Several drugs may cause glucose intolerance, including:
Before having the test, let your health care provider know if you are taking any of these medications.
American Diabetes Association. Standards of medical care in diabetes -- 2010. Diabetes Care. 2010;33:S11-S61.
Inzucchi SE, Sherwin RS. Type 2 diabetes mellitus. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 248.
Review Date: 5/23/2010
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