Oral Glucose Tolerance

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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 test for a diagnosis of prediabetes, 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 a diagnosis of prediabetse or 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 around 140 mg/dl (7.8 mmol/l) from 30 minutes to 1 hour after administration of the oral glucose dose, and should then return to near fasting levels of 100 mg/dl (5.6 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 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 100 gram 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 glucose levels peak below normal levels (i.e., below approximately 140 mg/dl or 7.8 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 mellitus.

Reviewed by Dr. Jason C. Baker, M.D. 05/11