You’ll fast for at least 8 hours -- but not more than 16 -- the night before you take the test. When you get to the doctor’s office, he’ll test your fastingblood sugar. Afterward, you’ll get 75 grams of glucose (100 grams for pregnant women), usually in the form of a sweet-tasting liquid that you drink. The doctor will take blood samples up to four times to measure your blood sugar levels.
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Obstetricians use several methods to do this test, but the one described here is standard.
How Reliable Is the Test?
For the oral glucose tolerance test to give reliable results, you have to be in good health. You can’t have any other illnesses -- not even a cold. And you can’t be taking drugs that could affect your blood sugar.
You'll have to watch your carbs beforehand, too. Don’t eat more than 150 to 200 grams a day for 3 days before the test. Don’t smoke or drink coffee the morning of the test, either.
What Does It Measure?
The classic oral glucose tolerance test measures blood sugar levels over a period of 3 hours. Some doctors simply get a beginning blood sample, followed by a sample 2 hours after you drink the glucose solution.
If you don’t have diabetes, your blood sugar levels will rise and then fall quickly. If you do have it, the levels rise higher than normal don’t come back down as fast.
If the test shows that your levels are between normal and diabetic, you have what doctors call impaired glucose tolerance (IGT). People with IGT don’t have diabetes yet.