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Diagnosing Impaired Glucose Tolerance (IGT)

People with IGT have blood glucose levels that are higher than normal but not high enough to say they have diabetes. This condition is diagnosed using the oral glucose tolerance test (OGTT). After a fast of 8 to12 hours, a person's blood glucose is measured before and 2 hours after drinking a glucose-containing solution.

  • In normal glucose tolerance, blood glucose rises no higher than 140 mg/dl 2 hours after the drink.
  • In impaired glucose tolerance (IGT), the 2-hour blood glucose is between 140 and 199 mg/dl.
  • If the 2-hour blood glucose rises to 200 mg/dl or above, a person has diabetes.

How does the fasting blood glucose test differ from the oral glucose tolerance test?

In the fasting blood glucose test, a person's blood glucose is measured after a fast of 8 to 12 hours:

  • A person with normal blood glucose has a blood glucose level below 100.
  • A person with impaired fasting glucose has a blood glucose level between 100 and 125 mg/dl.
  • If the fasting blood glucose level rises to126 mg/dl or above, a person has diabetes.

The OGTT includes measures of blood glucose levels after a fast and after a glucose challenge. In 1997, an American Diabetes Association (ADA) expert panel recommended that doctors use the fasting blood glucose test to screen their patients for diabetes because the test is easier and less costly than the OGTT. Though the fasting glucose test detects most diabetes cases, the OGTT is more sensitive in identifying people with blood glucose problems that may first appear only after a glucose challenge.

For a person with IGT, what is the risk of developing type 2 diabetes?

As few as 1 to as many as 10 of every 100 persons with IGT will develop diabetes per year. The risk of getting diabetes rises as people become more overweight and more sedentary, have a stronger family history of diabetes, and belong to an "at-risk" racial or ethnic minority group.

How many people in the U.S. have IGT?

About 20 million people in the U.S. have IGT, according to the National Health and Nutritional Examination Survey III. A recent study, the Diabetes Prevention Study, focused its efforts on preventing or delaying the development of diabetes only in people with glucose levels in the upper half of the impaired glucose tolerance range. They used an intensive lifestyle intervention program focused on diet and exercise that enabled participants to lose about 5-7 percent of their body weight and exercise about 30 minutes per day. This level of weight loss and increased exercise reduced the risk of developing type 2 diabetes by 58 percent. In a second arm of the study, participants taking a medication, metformin, reduced their risk by 31 percent. Since DPP researchers studied the interventions only in people with glucose levels in the upper half of the impaired glucose tolerance range, the findings can only be strictly applied to the 10 million people with similar glucose levels after an OGTT. However, many researchers think it is reasonable to assume that millions more people with IGT would also benefit from the DPP interventions.

See also: What Is Pre-Diabetes?

Adapted from: NIDDK, National Institutes of Health, Diabetes Prevention Program, Questions and Answers, August 2001.

Attention visitors from outside the US: You can easily convert US to non-US blood glucose levels by clicking here.

Page last updated: November 24, 2014