Glucose Tolerance Test
Medical Revision Author: Ruchi Mathur, MD
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR
What is the glucose tolerance test?
Though not routinely used anymore,
the oral glucose tolerance test (OGTT) is the gold standard for making the
diagnosis of type 2 diabetes. It is still commonly used for diagnosing
gestational diabetes. 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). 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 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 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- 200
grams per day). The morning of the test, the person should not smoke or drink
coffee.
What does the glucose tolerance test measure?
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, 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. In addition, some physicians advocate the use of medications, such as
metformin (Glucophage), to help prevent/delay the onset of overt diabetes.
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.
How are the results of the glucose tolerance test evaluated?
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 200 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 100g OGTT, a fasting plasma glucose of more
than 95 mg/dl, a 1-hour glucose level of more than 180 mg/dl, a 2-hour
glucose level of more than 155 mg/dl, or a 3-hour glucose level of more than
140 mg/dl.
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From the Doctors at MedicineNet.com  |
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- Diabetes - Information on Diabetes (Diabetes Mellitus) including types, causes, symptoms of diabetes, medications, and treatment. Produced by the doctors of MedicineNet.com Source:MedicineNet
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Last Editorial Review: 3/23/2005
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