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Oral Glucose Tolerance Test

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. 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 9 lb (4.1 kg).
    • Are younger than age 25 and were overweight before getting pregnant.
  • Diagnose prediabetes and diabetes.

How To Prepare

Glucose tolerance diagnostic test

To prepare for the glucose tolerance diagnostic test:

  • Eat a balanced diet that contains at least 150 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 doctor 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 doctor 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 tolerance diagnostic test

On the day of testing, the following steps will be done:

  • A blood sample will be collected when you arrive. This is your fasting blood glucose value. It provides a baseline for comparing other glucose values.
  • You will be asked to drink a sweet liquid containing a measured amount of glucose. It is best to drink the liquid quickly. For the standard glucose tolerance test, you will drink 75 grams or 100 grams. Pregnant women drink 75 grams of glucose.
  • Blood samples will be collected at timed intervals of 1, 2, and sometimes 3 hours after you drink the glucose. Blood samples may also be taken as soon as 30 minutes to more than 3 hours after you drink the glucose.

Blood test

The health professional taking a sample of your blood will:

  • Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
  • Clean the needle site with alcohol.
  • Put the needle into the vein. More than one needle stick may be needed.
  • Attach a tube to the needle to fill it with blood.
  • Remove the band from your arm when enough blood is collected.
  • Apply a gauze pad or cotton ball over the needle site as the needle is removed.
  • Apply pressure to the site and then a bandage.
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WebMD Medical Reference from Healthwise

Last Updated: July 05, 2011
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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