In the test, a person fasts overnight (at least 8 but not more than 16 hours). Then first, the fasting plasma glucose is tested. After this test, the person receives 75 grams of glucose (100 grams for pregnant women). 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.
For the 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 the person should not be taking medicines that could affect the blood glucose. For 3 days before the test, the person should have eaten a diet high in carbohydrates (150-200 grams per day). The morning of the test, the person should not smoke or drink coffee.
The oral glucose tolerance test measures blood glucose levels 5 times over a period of 3 hours. 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 what is termed "impaired glucose tolerance" (IGT). People with IGT do not have diabetes. Each year, only 1-5% of people whose test results show IGT actually develop diabetes. And with retesting, as many as half of the people with IGT have normal oral glucose tolerance test results. Weight loss and exercise may help people with IGT return their glucose levels to normal.
Oral glucose tolerance depends on a number of factors including the ability of the intestine to absorb glucose, the power of the liver to take up and store glucose, the capacity of the pancreas to produce insulin, the amount of "active" insulin it produces, and the sensitivity of the cells in the body to the action of insulin.
The outcome of the test may show:
- Normal glucose tolerance
- Abnormal glucose tolerance
- Depressed glucose tolerance -- in which the blood glucose peaks sharply before declining slower then usual to normal levels -- as in:
- Increased glucose tolerance -- in which the blood glucose levels peak at lower than normal levels -- as in the: