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What does the glucose tolerance test measure?
The classic oral glucose tolerance test measures blood glucose levels five times over a period of three hours. Some physicians simply take a baseline blood sample followed by a sample two hours after drinking the glucose solution. In a person without diabetes, the glucose levels rise and then fall quickly. In someone with diabetes, glucose levels rise higher than normal and fail to come back down as fast.
People with glucose levels between normal and diabetic levels have so-called impaired glucose tolerance (IGT). People with impaired glucose tolerance do not have diabetes.
Each year, 1% to 5% of people whose test results show impaired glucose tolerance actually develop diabetes. Weight loss and exercise may help people with impaired glucose tolerance return their glucose levels to normal. In addition, some physicians advocate the use of medications, such as metformin (Glucophage), to help prevent/delay the onset of overt diabetes. Studies have shown that impaired glucose tolerance itself may be a risk factor for the development of heart disease, and whether impaired glucose tolerance turns out to be an entity that deserves treatment itself is something that physicians are currently debating.
What is the preparation for a glucose tolerance test?
As mentioned previously, preparation for the oral glucose tolerance test involves fasting overnight (from 8 to 16 hours) and participating normally in activities of daily living. The individual should eat and drink as they normally do prior to the test. The morning of the test, the person should not consume caffeine or smoke.