What is glucose, and glucose tolerance testing?

How is diabetes mellitus diagnosed?

The fasting plasma glucose test is the preferred way to diagnose diabetes. It is easy to perform and convenient. After the person has fasted overnight (at least 8 hours), a single sample of blood is drawn and sent to the laboratory for analysis.

Normal fasting plasma glucose levels are less than 110 milligrams per deciliter (mg/dl). Fasting plasma glucose levels of more than 126 mg/dl on two or more tests on different days indicate diabetes. If the overnight fasting blood glucose is greater than 126 mg/dl on two different tests on different days, the diagnosis of diabetes mellitus is made.

A random blood glucose test can also be used to diagnose diabetes. Random blood samples (if taken shortly after eating or drinking) may be used to test for diabetes when symptoms are present. A blood glucose level of 300 mg/dl or higher indicates diabetes, but it must be reconfirmed on another day with a fasting plasma glucose or an oral glucose tolerance test.

What is the oral glucose tolerance test?

With an oral glucose tolerance test, the 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 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- 300 grams per day). The morning of the test, the person should not smoke or drink coffee.

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. 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, only 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. 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 physicians are currently debating.

What may the results of the oral glucose tolerance test indicate?

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

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 300 mg/dl.

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.

Diabetes: A person has diabetes when two diagnostic tests done on different days show that the blood glucose level is high.

Gestational diabetes: A woman has gestational diabetes when she has any two of the following: a fasting plasma glucose of more than 105 mg/dl, a 1-hour glucose level of more than 190 mg/dl, a 2-hour glucose level of more than 165 mg/dl, or a 3-hour glucose level of more than 145 mg/dl.

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