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Oral Glucose Tolerance Test

Brief Description – The oral glucose tolerance test (OGTT) is a laboratory test to check how your body breaks down sugar. There is a similar type of glucose tolerance test called intravenous glucose tolerance test.

Why the Test is Performed

Glucose is the sugar the body uses for energy. Patients with untreated diabetes have high blood glucose levels. Most often, the first tests used to diagnose diabetes in persons who are not pregnant are:

  •  Fasting blood glucose level: diabetes is diagnosed if it is higher than 126 mg/dL on two different tests
  •  Haemoglobin A1c test: diabetes is diagnosed if the test result is 6.5% or higherGlucose tolerance tests are also used to diagnose diabetes. The oral glucose tolerance test is used to screen for, or diagnose diabetes in people with a fasting blood glucose level that is high, but is not high enough (above 125 mg/dL) to meet the diagnosis for diabetes.

    How the Test is Performed

    Before the test begins, a sample of blood will be taken. You will then be asked to drink a liquid containing a certain amount of glucose (usually 75 grams). Your blood will be taken again every 30 to 60 minutes after you drink the solution. The test may take up to 3 hours.

    How to Prepare For the Test

    Make sure you eat normally for several days before the test. Do not eat or drink anything for at least 8 hours before the test. You cannot eat during the test. Serious stress to the body, such as trauma, stroke, heart attack, or surgery can raise your blood glucose level.

    Vigorous exercise can lower your blood glucose level. Some drugs can raise or lower your blood glucose level. Before having the test, tell your doctor about any drugs you are taking.

    During the Test

    Drinking the glucose solution is similar to drinking very sweet soda. With the blood test, some people feel nauseated, sweaty, light-headed, or may even feel short of breath or faint after drinking the glucose. Tell your doctor if you have a history of these symptoms related to blood tests or medical procedures. When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing.

After the Test

You will need to rest after the test especially if you experienced any of the symptoms listed above. Ask your doctor when you can resume your medications and if you need to modify your diet.


Serious side effects from this test are very rare. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

  •  Excessive bleeding
  •  Fainting or feeling light-headed
  •  Hematoma (blood accumulating under the skin)
  •  Infection (a slight risk any time the skin is broken)

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