The Oral Glucose Tolerance Test

A Test to Diagnose Diabetes

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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 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. According to the Diabetes Standards of Care, the test is also preferred in diagnosing Type 2 diabetes in adolescents and children. 

How Is the Test Conducted?

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 of 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. The test can be done using a one-step approach of ingesting 75g of glucose or using a two-step approach which first screens for glucose intolerance using 50g of glucose. If a person fails the challenge they will follow up with 100-gram glucose tolerance test. The advantage of this is that you do not have to be fasting to take the 50g challenge test (which is more convenient), however, if you fail you must follow up and do the 100g test which requires you to be fasting.  

For the 75-gram oral glucose tolerance test that is used to screen for gestational diabetes: 

The diagnosis of GDM is made when any of the following plasma glucose values are met or

  • Fasting: 92 mg/dL (5.1 mmol/L)
  • 1 h: 180 mg/dL (10.0 mmol/L)
  • 2 h: 153 mg/dL (8.5

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

A person will have to take a 100g glucose tolerance test if blood sugar is:

  • 1 hour: greater than or equal to 140 mg/dL

For the 100-gram oral glucose tolerance test:

The diagnosis of GDM is made when any of the following plasma glucose values are met or

  • Fasting: 95 mg/dL
  • 1 hour: 180 mg/dL
  • 2 hours: 155 mg/dL
  • 3 hours: 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.

View Article Sources
  • "Glucose Tolerance Test." University of Maryland Medical Center. 11 Aug. 2006. University of Maryland Medical Center. 
  • American Diabetes Association. Standards of Care. Diabetes Care 2016;39(Suppl. 1):S1–S119
  • Committee on Practice Bulletins. Obstetrics. Practice Bulletin No. 137: Gestational diabetes mellitus. Obstet Gynecol 2013; 122:406.
  • "Oral Glucose Tolerance Test During Pregnancy." The Cleveland Clinic Health Information Center. 31 Oct. 2006. The Cleveland Clinic. 
  • "Glucose Tolerance Test." Medline Plus Medical Encyclopedia. 11 Aug. 2006. U.S. National Library of Medicine and the National Institutes of Health.