Glucose Tolerance Test
Melissa Conrad Stöppler, MD
Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.
William C. Shiel Jr., MD, FACP, FACR
Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.
What is the glucose tolerance test?
Though not routinely used anymore, the oral glucose tolerance test (OGTT) has been considered to be 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 75 grams of glucose. 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.
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 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.
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 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.
Learn more about: Glucophage
What is the preparation for a glucose tolerance test?
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.
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