An oral glucose tolerance test is usually only done to diagnose gestational diabetes. For an OGTT, you must be in good health, not be taking any medications that might affect glucose, and be normally active. You should not take this test if you have been prescribed bed rest by your doctor. You should discuss this test with both your regular doctor and your obtetrician.
The test measures your blood glucose five times over a three-hour period. If your body's response to glucose is normal, then during the test your blood glucose will rise and then fall quickly. If you have diabetes, your blood glucose will rise higher than normal and stay high during the test.
The OGTT takes some time and preparation.
- Your doctor may have you eat a diet rich in carbohydrates, such as breads, desserts, grains and pasta, for about three days. (Not all doctors agree that this is necessary; many will recommend simply eating a balanced diet.)
- You'll fast overnight — about 12 hours — before the test. You can't eat, smoke or drink anything, including coffee, before testing.
- Your doctor will take a blood sample. It will be analyzed for glucose content.
- You'll consume 75 grams of glucose — or 100 grams if you're pregnant — in a sugary drink.
- During the test, you must sit or lie quietly.
- Your doctor will take additional blood samples — 1 hour after you are given the drink, then two hours later, and then (for the 100 gram test) three hours later. Samples may be taken in between these times, too. Each sample is analyzed for the level of blood glucose.
Your doctor will compare your results against the normal levels for glucose tolerance. Normal oral glucose tolerance depends on whether or not you are pregnant.
The 75-gram test (taken if you're not pregnant)
- If your glucose at the two-hour test mark measures 200 mg/dL or more, your doctor will need to do a different type of test on a different day to verify the results. If your second test results are high again, you have diabetes.
- If your blood glucose at the two-hour test mark measures 140 mg/dL or greater but less than 200 mg/dL on two tests, you have an elevated blood glucose condition called impaired glucose tolerance, or IGT. This is also called prediabetes.
The 100-gram test (taken if you're pregnant)
- Normal glucose levels are less than 95 mg/dL before the test, less than 180 mg/dL at one hour, less than 155 mg/dL at two hours, and less than 140 ml/dL after three hours.
- If your levels are higher than normal at two or more of these points, you are likely to have gestational diabetes.
Your doctor will confirm all abnormal tests with a second test before diagnosing diabetes.
Written by award-winning health writer Bobbie Hasselbring
Reviewed by Beth Seltzer, MD
Last updated June 2008