The oral glucose tolerance test (OGTT) measures the body's ability to use a type of sugar, called glucose, that is the body's main source of energy. An OGTT can be used to diagnose prediabetes and diabetes. An OGTT is most commonly done to check for diabetes that occurs with pregnancy (gestational diabetes).

Why It Is Done

The oral glucose tolerance test (OGTT) is done to:

• Check pregnant women for gestational diabetes. When done for this purpose, the test is called a glucose challenge screening test, and it is usually done during the 24th to the 28th week of pregnancy. You have an increased chance of developing gestational diabetes if you:
  • Have had gestational diabetes during a previous pregnancy.
  • Have previously given birth to a baby who weighed more than 8.8lb.
  • Are younger than age 25 and were overweight before getting pregnant.
• Confirm the presence of gestational diabetes if other blood glucose measurements are high.
• Screen women who have polycystic ovary syndrome (PCOS) for diabetes.
• Diagnose prediabetes and diabetes.

How To Prepare

Glucose challenge screening test for gestational diabetes

No preparation is usually needed for the screening test done during pregnancy. This test can be done at any time of the day, so, you do not need to limit food or fluids before the test.

Glucose tolerance diagnostic test

To prepare for the glucose tolerance diagnostic test:

• Eat a balanced diet that contains at least 150 to 200 grams (g) of carbohydrate per day for 3 days before the test. Fruits, breads, cereals, grains, rice, crackers, and starchy vegetables such as potatoes, beans, and corn are good sources of carbohydrate.
• Do not eat, drink, smoke, or exercise strenuously for at least 8 hours before your first blood sample is taken.
• Tell your health professional about all prescription and nonprescription medicines you are taking. You may be instructed to stop taking certain medicines before the test.

The glucose tolerance diagnostic test may take up to 4 hours. Since activity can interfere with test results, you will be asked to sit quietly during the entire test. Do not eat during the test. You may drink only water during this time.

Talk to your health professional about any concerns you have regarding the need for the test, its risks, or how it will be done. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

How It Is Done

Glucose challenge screening test for gestational diabetes

• You will be asked to drink a sweet liquid containing glucose. It is best to drink the liquid quickly.
• A blood sample will be collected 1 hour after you drink the glucose. Normally, blood glucose levels peak within an hour and then begin to drop.