Oral Glucose Tolerance

Blood Tests for Diabetes Diagnosis: Oral Glucose Tolerance Test


Also known as: OGTT; 3-hour glucose tolerance test.

What is it? A test that measures blood glucose levels four to five times over a 3-hour period. The patient is administered an oral dose of glucose solution (75 to 100 grams of an extremely sweet drink), which should cause glucose levels to rise in the first hour, and then fall back to normal within three hours as the body produces insulin to normalize glucose levels.

Why is this test performed? To confirm a diagnosis of diabetes or gestational diabetes (and to diagnose other metabolic diseases). Because the OGTT is a more sensitive test than the fasting plasma glucose test, and involves multiple blood draws to monitor insulin production, it can often detect cases of diabetes that may be missed by the fasting test.

How frequently should this test be performed? Once, in select individuals, for diagnosis of diabetes. A second retest may be appropriate in some patients, particularly pregnant women being tested for gestational diabetes. Additional laboratory tests may be required for an accurate diagnosis.

What is the "normal" range for results? Laboratory test methods and measurements vary in this test. The most commonly used test protocols include the Wilkerson point system; the Fajans-Conn system, or the National Institutes of Health (NIH) system. On average, normal glucose levels typically peak at 160 180 mg/dl (8.89 10 mmol/l) from 30 minutes to 1 hour after administration of the oral glucose dose, and should then return to fasting levels of 140 mg/dl (7.78 mmol/l) or less within a 2- to 3-hour period.

Factors such as age, weight, and race can influence results, as can recent illnesses and certain medications. For example, older individuals will have an upper limit increase of 1 mg/dl in glucose tolerance for every year over age 50. For more information, ask your doctor what protocol your lab uses.

What do abnormal results mean? Glucose levels that quickly rise above normal levels (i.e., 200 mg/dl / 11.11 mmol/l or higher) and take longer to normalize usually indicate diabetes mellitus. Pregnant women with a 1-hour glucose level of over 180 mg/dl (10 mmol/l), a 2-hour level of over 155 mg/dl (8.6 mmol/l), or a 3-hour level of over 140 mg/dl (7.78 mmol/l) may have gestational diabetes. A follow-up test may be required.

If fasting glucose levels peak below normal levels (i.e., approximately 140 mg/dl or 7.78 mmol/l), a condition that indicates increased glucose tolerance, such as Addison's disease, hypothyroidism, malabsorption, or hypopituitarism may be present.

OGTT Fast Facts from the NIH:

  • In normal glucose tolerance, blood glucose rises no higher than 140 mg/dl (7.78 mmol/l) 2 hours after the drink.
  • In impaired glucose tolerance (IGT), or pre-diabetes, the 2-hour blood glucose is between 140 and 199 mg/dl (7.78 mmol/l and 11.1 mmol/l).
  • If the 2-hour blood glucose rises to 200 mg/dl (11.11 mmol/l) or above, a person has diabetes.

Last Modified Date: May 17, 2013

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by Brenda Bell
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