The glucose tolerance test is a lab test to check how your body breaks down sugar.
Tests to screen for diabetes during pregnancy are done differently.
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 may take up to 3 hours.
A similar test is the intravenous (IV) glucose tolerance test (IGTT). It is rarely used, and is never used to diagnose diabetes. In one version of the IGTT, 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. This IGTT is almost always used for research purposes only.
How the Test will Feel
Drinking the glucose solution is similar to drinking very sweet soda.
Serious side effects from this test are very uncommon. With the blood test, some people feel nauseated, sweaty, lightheaded, 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 blood tests or medical procedures.
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 or a slight bruise. This soon goes away.
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.
Normal blood values for a 75 gram OGTT used to check for type 2 diabetes in those who are not pregnant:
- Fasting: 60 to 100 mg/dL (3.3 to 5.5 mmol/L)
- 1 hour: less than 200 mg/dL (11.1 mmol/L)
- 2 hours: less than 140 mg/dL (7.8 mmol/L)
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.
You may have some of the symptoms listed above under the heading titled "How the Test will Feel."
Veins and arteries vary in size from one person 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 lightheaded
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
What Abnormal Results Mean
A glucose level that is higher than normal may mean you have pre-diabetes or diabetes:
- A 2 hour value between 140 and 200 mg/dL (7.8 and 11.1 mmol/L) is called impaired glucose tolerance. Your doctor may call this "pre-diabetes." It means you are at increased risk of developing diabetes over time.
- A glucose level of 200 mg/dL (11.1 mmol/L) or higher is used to diagnose diabetes.
Serious stress to the body, such as from 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 provider about any medicines you are taking.
Why the Test is Performed
Glucose is the sugar the body uses for energy. People with untreated diabetes have high blood glucose levels.
Most often, the first tests used to diagnose diabetes in people who are not pregnant are:
- Fasting blood glucose level: diabetes is diagnosed if it is higher than 126 mg/dL (7 mmol/L) on 2 different tests
- Hemoglobin A1c test: diabetes is diagnosed if the test result is 6.5% or higher
Glucose tolerance tests are also used to diagnose diabetes. The OGTT is used to screen for, or diagnose diabetes in people with a fasting blood glucose level that is high, but is not high enough (above 125 mg/dL or 7 mmol/L) to meet the diagnosis for diabetes.
American Diabetes Association. Standards of medical care in diabetes -- 2017: 2. Classification and diagnosis of diabetes. Diabetes Care. 2017;40(Suppl 1):S11-S24. PMID: 27979889
Chernecky CC, Berger BJ. Glucose tolerance test (GTT, OGTT) - blood. In: Chernecky CC, Berger BJ, eds. Laboratory Tests and Diagnostic Procedures. 6th ed. Philadelphia, PA: Elsevier Saunders; 2013:591-593.