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Oral Glucose Tolerance Test

Test Overview

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.

How It Feels

You may find it hard to drink the extremely sweet glucose liquid. Some people feel sick after drinking the glucose liquid and may vomit. Vomiting may prevent you from completing the test on that day.

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

You may feel faint from having several blood samples taken in one day. But the amount of blood taken will not cause significant blood loss or anemia.

Risks

Some people's blood glucose levels drop very low toward the end of the test. But some people feel like their sugar levels are low, when their levels actually are not low. Symptoms of low blood glucose include weakness, hunger, sweating, and feeling nervous or restless. If you develop these symptoms during the test, you may have your sugar level checked quickly with a glucose meter. If your level is very low, the test will be stopped.

There is very little risk of a problem from having blood drawn from a vein.

  • You may develop a small bruise at the puncture site. You can reduce the risk of bruising by keeping pressure on the site for several minutes after the needle is withdrawn.
  • In rare cases, the vein may become inflamed after the blood sample is taken. This condition is called phlebitis and is usually treated with a warm compress applied several times daily.
  • Continued bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can also make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your health professional before your blood is drawn.

Results

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.

Normal

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what’s normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

But with screening for gestational diabetes, the American Diabetes Association has recommended specific glucose values be used for diagnosis. If any of your glucose values are higher than what is listed in the table, you will be diagnosed with gestational diabetes.

Glucose tolerance diagnostic test (for gestational diabetes)1
75 g of glucose Fasting:

Less than 95 mg/dL or 5.3 mmol/L
1-hour:

Less than 180 mg/dL or 10.0 mmol/L
2-hour:

Less than 153 mg/dL or 8.5 mmol/L

You have prediabetes if the results of your oral glucose tolerance test are 140 to 199 mg/dL (2 hours after the beginning of the test).

High values

High glucose levels may be caused by:

Low values

Low glucose levels may be caused by:

  • Certain medicines, such as medicines used to treat diabetes, some blood pressure medicines (such as propranolol), and some medicines for depression (such as isocarboxazid).
  • Decreased production of the hormones cortisol and aldosterone (Addison's disease).
  • Problems with the thyroid gland (hypothyroidism) or an underactive pituitary gland.
  • A tumor or other problems of the pancreas.
  • Liver disease.

Many conditions can change blood glucose levels. Your doctor will discuss any significant abnormal results with you in relation to your symptoms and past health.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • Smoking.
  • Taking certain medicines, such as corticosteroids, diuretics, seizure medicines, birth control pills, nonsteroidal anti-inflammatory drugs (NSAIDs), and some medicines used to treat high blood pressure.
  • Alcohol.
  • Recent surgery, illnesses, and infectious diseases.
  • Weight loss through dieting.
  • Long periods of bed rest (such as from a hospitalization or illness).

What To Think About

If you are diagnosed with gestational diabetes, you have a higher risk of developing diabetes later in life. The American Diabetes Association recommends screening for diabetes and prediabetes every three years.2

References

Citations

  1. American Diabetes Association (2011). Diagnosis and classification of diabetes mellitus. Diabetes Care, 34(Suppl 1): S62–S69.
  2. American Diabetes Association (2011). Standards of medical care in diabetes – 2011. Diabetes Care, 34(Suppl 1): S11–S61.

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.

Credits

By Healthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer Alan C. Dalkin, MD - Endocrinology
Last Revised July 5, 2011

Last Revised: July 5, 2011

Author: Healthwise Staff

Medical Review: E. Gregory Thompson, MD - Internal Medicine & Alan C. Dalkin, MD - Endocrinology

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