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Oral Glucose Tolerance Test

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What is the glucose tolerance test?

Though not routinely used anymore, the oral glucose tolerance test (OGTT) is the gold standard for making the diagnosis of type 2 diabetes. However, it is still commonly used for diagnosing gestational diabetes. 

With an oral glucose tolerance test, the person fasts overnight (at least eight but not more than 16 hours). Then, the fasting plasma glucose is tested. After this test, the person receives 75 grams of glucose (100 grams for pregnant women). There are several methods used by obstetricians to do this test, but the one described here is standard. Usually, the glucose is in a sweet-tasting liquid that the person drinks. Blood samples are taken up to four times to measure the blood glucose.

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How reliable is the glucose tolerance test?

For the glucose tolerance test to give reliable results, the person must be in good health (not have any other illnesses, not even a cold). Also, the person should be normally active (not lying down, for example, as an inpatient in a hospital) and should not be taking drugs that could affect the blood glucose. For three days before the test, the person should not eat a diet high in carbohydrates (150- 200 grams per day). The morning of the test, the person should not smoke or drink coffee.

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 doctors simply get a beginning 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 have impaired glucose tolerance (IGT). People with IGT do not have diabetes. Each year, 1%-5% of people whose test results show IGT actually develop diabetes. Weight loss and exercise may help people with IGT return their glucose levels to normal. In addition, some doctors advocate the use of medications, such as metformin (Glucophage), to help prevent or delay the onset of overt diabetes. Recent studies have shown that IGT itself may be a risk factor for the development of heart disease, and whether IGT turns out to be an entity that deserves treatment itself is something that is currently debated.

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