Diseases

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

• Normal response: A person is said to have a normal response when the two hour glucose level is less than 140 mg/dl, and all values between 0 and 2 hours are less than 200 mg/dl.
• Impaired glucose tolerance (IGT): A person is said to have impaired glucose tolerance when the fasting plasma glucose is less than 126 mg/dl and the two hour glucose level is between 140 and 199 mg/dl. This is sometimes referred to as "prediabetes" because people with IGT have a higher risk of developing diabetes.
• Diabetes: A person has diabetes when two diagnostic tests done on different days show that the blood glucose level is high. This means either the two hour levels is greater than 200 mg/dl or the fasting glucose is noted as greater than 126 mg/dl. A glycosylated hemoglobin (HbA1c) level of 6.5% or more also supports a diagnosis of diabetes mellitus.
• Diabetes during pregnancy: A pregnant woman has diabetes if she has a fasting plasma glucose of over 92 mg/dl, or a two hour glucose level greater than 153 mg/dl.

For the glucose tolerance test to give reliable results, the person must be in good health (not have any other illnesses, not even a common 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. In preparation for the oral glucose tolerance test, the person should eat and drink as they normally would. The morning of the test, the person should not smoke or consume caffeine.

As mentioned previously, the glucose tolerance test is used for the diagnosis of gestational diabetes (diabetes that develops during pregnancy). It may be used if there are equivocal fasting or random blood glucose results, or to screen for gestational diabetes in pregnant women between 24 to 28 weeks of gestation who are not known to have diabetes. The test may also be used in the postpartum period to detect diabetes in women who had gestational diabetes during pregnancy. Women who had gestational diabetes do not always develop diabetes later in life, but they should undergo diabetes screening at least every three years throughout their life.

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.

Though no longer routinely used for diagnosing diabetes. The oral glucose tolerance test (OGTT) was the gold standard for making the diagnosis of type 2 diabetes. It is still commonly used during pregnancy for diagnosing gestational diabetes. With an oral glucose tolerance test, the person fasts overnight (at least 8 hours, but not more than 16 hours). The next morning, the fasting plasma glucose is tested. After this test, the person receives a dose of oral glucose (the dose depends upon the length of the test). There are several methods employed 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 at different time points after consumption of the sugar to measure the blood glucose.

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.