The oral glucose tolerance test:

How to prepare for the test:

With an oral glucose tolerance test, the person fasts overnight (at least 8 but not more than 16 hours). Then the fasting plasma glucose is tested. After this test, the person receives 75 grams of glucose. The classic oral glucose tolerance test measures blood glucose levels 5 times over a period of 3 hours. Some physicians simply get a baseline blood sample followed by a sample 2 hours after drinking the glucose solution.

1. In a person without diabetes, the glucose levels rise and then fall quickly.
2. In diabetics, glucose levels rise higher than normal and fail to come back down as fast.
3. 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.

Evaluating the results of the oral glucose tolerance test:

Glucose tolerance tests may lead to one of the following diagnoses:

1. Normal response: A person is said to have a normal response when the 2-hour glucose level is less than 140 mg/dl, and all values between 0 and 2 hours are less than 200 mg/dl.
2. Impaired glucose tolerance: A person is said to have IGT when the fasting plasma glucose is less than 126 mg/dl and the 2-hour glucose level is between 140 and 199 mg/dl.
3. Diabetes: A person has diabetes when two diagnostic tests done on different days show that the blood glucose level is high.

Time in hours

Glucose tolerance curve

In normal glucose tolerance curve; fasting level = 65-120mg/dl. The ascending limb of the curve represents glucose utilization. Maximum absorption occurs in one hour (level reaches 1.5 fasting level). The descending limb of the curve represents glucose utilization by the tissues in response to insulin secretion. The blood glucose level returns to fasting level in 2 hours.