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Glucose tolerance test
Definition
The glucose tolerance test is a lab test to check how your body breaks down sugar.
Alternative Names
Oral glucose tolerance test
How the Test is Performed
The most common glucose tolerance test is the oral glucose tolerance test (OGTT).
Before the test begins, a sample of blood will be taken. For information on how this is done, see: Venipuncture
You will then be asked to drink a liquid containing a certain amount of glucose (usually 75 grams). Your blood will be taken again every 30 to 60 minutes after you drink the solution.
The test takes up to 3 hours.
A similar test is the IV glucose tolerance test (IGTT). It is rarely used, and never used to diagnose diabetes. 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. However, the timing may vary.
How to Prepare for the Test
Make sure you eat normally for several days before the test.
Do not eat or drink anything for at least 8 hours before the test. You cannot eat during the test.
Ask your health care provider if any of the medicines you take can affect the test results.
How the Test Will Feel
Some people feel nauseated, sweaty, light-headed, or may even feel short of breath or faint after drinking the glucose. However, serious side effects of this test are very uncommon.
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.
Why the Test is Performed
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 blood 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 when the disease is suspected, even though the fasting blood glucose level is normal.
Normal Results
Normal blood values for a 75-gram oral glucose tolerance test used to check for type 2 diabetes in those who are not pregnant:
- Fasting: 60 -100 mg/dL
- 1 hour: less than 200 mg/dL
- 2 hours: less than 140 mg/dL
Note: mg/dL = milligrams per deciliter
The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
What Abnormal Results Mean
Higher-than-normal levels of glucose may mean you have prediabetes, diabetes, or gestational diabetes.
Between 140 - 200 mg/dL is called impaired glucose tolerance. Your doctor may call this "prediabetes." It means you are at increased risk for developing diabetes.
A glucose level of 200 mg/dL or higher is a sign of diabetes.
However, high glucose levels may be related to another medical problem (for example, Cushing syndrome).
Risks
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:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Considerations
Factors that may affect the test results:
- Acute stress (for example, from surgery or an infection)
- Vigorous exercise
Several drugs may cause glucose intolerance, including:
- Atypical antipsychotic medications, including aripiprazole, olanzapine, quetiapine, risperidone, and ziprasidone
- Beta-blockers (for example, propranolol)
- Birth control pills
- Corticosteroids (for example, prednisone)
- Dextrose
- Epinephrine
- Estrogens
- Glucagon
- Isoniazid
- Lithium
- Phenothiazines
- Phenytoin
- Salicylates (including aspirin)
- Thiazide diuretics (for example, hydrochlorothiazide)
- Triamterene
- Tricyclic antidepressants
Before having the test, let your health care provider know if you are taking any of these medications.
References
American Diabetes Association. Standards of medical care in diabetes--2012. Diabetes Care. 2011 Jan;35 Suppl 1:S11-63.
Buse JB, Polonsky KS, Burant CF. Type 2 diabetes mellitus.In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, Larsen PR, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, Pa: Saunders Elsevier;2011:chap 31.
Inzucchi SE, Sherwin RS. Type 2 diabetes mellitus. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 237.
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.
