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Oral Glucose Tolerance Test

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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).

But with screening for gestational diabetes in pregnant women, the American Diabetes Association has recommended specific glucose values be used for diagnosis.

Glucose tolerance diagnostic test (for gestational diabetes)3
 Values that indicate gestational diabetes.
75 g of glucose Fasting:

More than or equal to 92 mg/dL or 5.1 mmol/L

1-hour:

More than or equal to 180 mg/dL or 10.0 mmol/L

2-hour:

More than or equal to 153 mg/dL or 8.5 mmol/L

100 g of glucose3-hour:

More than or equal to 140 mg/dL or 7.8 mmol/L

High values

High glucose levels may be caused by:

  • Diabetes.
  • Gestational diabetes.
  • Hyperthyroidism.
  • Some medicines, such as corticosteroids, niacin, phenytoin (Dilantin), some diuretics, and some medicines used to treat high blood pressure, HIV, or AIDS.
  • Large amounts of the hormone cortisol in the blood (Cushing's syndrome).
  • Inherited diseases, such as hemochromatosis.
  • Pheochromocytoma.

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 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.
  • 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

When screening for gestational diabetes, a test might be done before the 100-gram oral glucose tolerance test (OGTT). For the first test, you will not need to fast. You will drink 50 grams of glucose. Your blood sample will be taken 1 hour later. If your glucose level is high—for example, over 140 mg/dL (7.8 mmol/L)—you will be asked to take the second test, the 100-gram OGTT.

Even though your gestational diabetes will probably go away after your baby is born, you are at risk for gestational diabetes if you become pregnant again and for type 2 diabetes later in life. You may also have a follow-up glucose tolerance test 6 to 12 weeks after your baby is born or after you stop breast-feeding your baby. If the results of this test are normal, you will still need to be screened for diabetes at least every 3 years.

WebMD Medical Reference from Healthwise

Last Updated: January 28, 2014
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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