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The Oral Glucose Tolerance Test

Can Help Prevent Diabetes

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Updated November 04, 2008

What is the Oral Glucose Tolerance Test?

The oral glucose tolerance test (OGTT), also referred to as the glucose tolerance test, measures the body’s ability to metabolize glucose, or clear it out of the bloodstream. The test can be used to diagnose diabetes, gestational diabetes (diabetes during pregnancy) or prediabetes (a condition characterized by higher-than-normal blood sugar levels that can lead to type 2 diabetes).

According to the National Institutes of Health (NIH), the OGTT it is better able to diagnose high blood glucose after a glucose challenge than the fasting blood glucose test. A doctor may recommend it if he or she suspects diabetes in cases where a patient’s fasting blood glucose level is normal. However, the test is more time-consuming and complicated than the fasting blood glucose test.

How Is the Test Conducted?

Those taking the OGTT to diagnose diabetes or prediabetes who are not pregnant should eat a normal, balanced diet the week before taking the test. Experts recommend that this diet include at least 150 to 200 grams of carbohydrates per day. Patients must fast for at least 8 to 12 hours before having the test.

After fasting, blood is drawn to establish a fasting glucose level. Next, a patient must quickly drink a sugary (glucose-rich) beverage. Typically, the drink contains 75 grams of carbohydrates, although other amounts are possible. Blood will be drawn at various intervals to measure glucose levels, usually one hour and two hours after the beverage is consumed.

What Does the Test Indicate?

The test reveals how quickly glucose is metabolized from the bloodstream for use by cells as an energy source. The normal rate of glucose clearing depends on the amount of glucose ingested. After fasting, the normal blood glucose rate is 60 to 100 mg/dL (milligrams per deciliter).

For 75 grams of glucose, normal blood glucose values are:

  • 1 hour: less than 200 mg/dL
  • 2 hours: less than 140 mg/dL. Between 140-200 mg/dL indicates impaired glucose tolerance (prediabetes). If test results are in this range, a patient is at an increased risk for developing diabetes. Greater than 200 mg/dL indicates diabetes.

Pregnancy and the Oral Glucose Tolerance Test

Pregnancy affects a woman’s ability to metabolize blood sugar. This is why the American Diabetes Association recommends an oral glucose tolerance test, which checks for gestational diabetes, for all expectant mothers. This test is common during the 24th to the 28th week of pregnancy. Typically, the dose of glucose that is given is 50 or 100 grams. Normal values for pregnancy are described below. Values above this range indicate gestational diabetes:

For the 50-gram oral glucose tolerance test that is used to screen for gestational diabetes:

  • 1 hour: less than 140 mg/dL
For the 100-gram oral glucose tolerance test:
  • Fasting: less than 95 mg/dL
  • 1 hour: less than 180 mg/dL
  • 2 hours: less than 155 mg/dL
  • 3 hours: less than 140 mg/dL

What’s Next?

Abnormally high levels of glucose may indicate diabetes, gestational diabetes or prediabetes. However, a patient will probably be asked to undergo another glucose screening test to confirm a diagnosis.

Remember that the oral glucose tolerance test is used not only to diagnose diabetes, but also to help provide information about the body’s ability to metabolize blood sugar. Higher values are likely to reflect diet and lifestyle issues as well as problems of insulin functioning. In this sense, this test is a signal for action that will help a patient take control of his or her health.

Sources:

"Glucose Tolerance Test." University of Maryland Medical Center. 11 Aug. 2006. University of Maryland Medical Center. 9 Sep. 2007. <http://www.umm.edu/ency/article/003466.htm>.

"Oral Glucose Tolerance Test During Pregnancy." The Cleveland Clinic Health Information Center. 31 Oct. 2006. The Cleveland Clinic. 9 Sep. 2007. <http://www.clevelandclinic.org/health/health-info/docs/2700/2799.asp?index=9696>.

"Glucose Tolerance Test." Medline Plus Medical Encyclopedia. 11 Aug. 2006. U.S. National Library of Medicine and the National Institutes of Health. 9 Sep. 2007.

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