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

If the screening test results indicate that you may have gestational diabetes, the complete glucose tolerance diagnostic test may be 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 to 100 grams; pregnant women drink 100 grams of glucose.
  • Blood samples will be collected at timed intervals of 1, 2, and 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.

Glucose challenge screening test for polycystic ovary syndrome (PCOS)

  • 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 2 hours hour after you drink the glucose. Normally, blood glucose levels peak within an hour and then begin to drop.

If the screening test results indicate that you may have diabetes, other tests may be needed.

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 difficult 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 as though 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

Normal glucose levels in a blood sample taken from your vein (called a plasma glucose value) may vary from lab to lab.

Glucose challenge screening test (for gestational diabetes)
50 grams (g) of glucose 1-hour:

Less than 140 milligrams per deciliter (mg/dL) or 7.8 millimoles per liter (mmol/L)

If values are between 140–180 mg/dL (7.8–10 mmol/L), a glucose tolerance diagnostic test may be recommended.

 

Glucose tolerance diagnostic test (for gestational diabetes)
100 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 155 mg/dL or 8.6 mmol/L

3-hour:

Less than 140 mg/dL or 7.8 mmol/L

 

Glucose tolerance screening test (for diabetes in women who have polycystic ovary syndrome)
75 g of glucose 2-hour:

Less than 140 mg/dL or 7.8 mmol/L

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).
  • A condition that prevents the intestines from absorbing nutrients from food, such as celiac disease.
  • Decreased production of the hormones cortisol and aldosterone (Addison's disease).
  • Problems with the thyroid gland (hypothyroidism) or an underactive pituitary gland.
  • A tumor of the pancreas (insulinoma).
  • Inflammation and scarring of the liver (cirrhosis).

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:

  • 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, heart attack, or childbirth.
  • A low-carbohydrate diet.
  • Vomiting during the test.
  • Emotional stress.
  • Fever and infection.

What To Think About

  • The oral glucose tolerance test is not commonly used to diagnose diabetes in people who are not pregnant. The American Diabetes Association prefers the fasting blood glucose test to diagnose diabetes.1 But a fasting blood glucose test may not identify some people with prediabetes or early diabetes. Many experts recommend using an oral glucose tolerance test if the result of a fasting blood glucose test is between 100 mg/dL (5.6 mmol/L) and 125 mg/dL (6.9 mmol/L). For more information, see the medical test Blood Glucose.
  • The glucose liquid may be replaced with a carbonated drink that contains glucose, such as soda pop, or a gelatin that contains glucose. These forms of glucose may be easier to drink or eat than a plain glucose solution.
  • Glucose tolerance test screening by age 30 is recommended for all women who have polycystic ovary syndrome.2 For more information, see the topic Polycystic Ovary Syndrome (PCOS).

References

Citations

  1. American Diabetes Association (2009). Standards of medical care in diabetes. Clinical Practice Recommendations 2009. Diabetes Care, 32(Suppl 1): S13–S61.
  2. American Association of Clinical Endocrinologists (2005). Position statement on metabolic and cardiovascular consequences of polycystic ovary syndrome. Endocrine Practice: 11(2): 126–134.

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 (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.

Credits

Author Christine Wendt, R.D., L.D.
Editor Susan Van Houten, RN, BSN, MBA
Associate Editor Maria Essig
Primary Medical Reviewer Caroline S. Rhoads, MD - Internal Medicine
Specialist Medical Reviewer David C.W. Lau, MD, PhD, FRCPC - Endocrinology & Metabolism
Last Updated July 14, 2009

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