The glucose tolerance test is a lab test to check how your body breaks down sugar.
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.
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. 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. Tell your doctor if you have a history of these symptoms related to eating sugar. 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. 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.
A blood glucose level that is higher than normal can be used to diagnose type 2 diabetes or high blood glucose during pregnancy (gestational diabetes). Insulin level 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 fasting blood glucose level is normal.
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 to 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
A glucose level than is higher than normal may mean you have prediabetes, diabetes, or gestational diabetes.
Between 140 and 200 mg/dL is called impaired glucose tolerance. Your doctor may call this "prediabetes." It means you are at increased risk of developing diabetes over time.
A glucose level of 200 mg/dL or higher is a sign of diabetes.
A high glucose level may be related to another medical problem (for example, Cushing syndrome).
You may have some of the symptoms listed above under the heading titled "How the test may feel."
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:
Fainting or feeling light-headed
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)
Stress due to, for example, trauma, stroke, heart attack, or surgery can raise your blood glucose level. Vigorous exercise can lower your blood glucose level.
Some medicines can raise or lower your blood glucose level. Before having the test, tell your health care provider if you are taking any medicines.
Medicines that can cause glucose intolerance, include:
Beta-blockers (such as propranolol)
Certain antipsychotic drugs
Corticosteroids (such as prednisone)
Oral contraceptives (birth control pills)
Salicylates (including aspirin)
Thiazide diuretics (such as hydrochlorothiazide)
American Diabetes Association. Standards of medical care in diabetes--2013. Diabetes Care. 2013 Jan;36 Suppl 1:S11-66.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, Bethanne Black, Stephanie Slon, and Nissi Wang.