Oral Glucose Tolerance Test


Women who have a blood sugar level greater than 140 mg/dl (7.8 mmol/l) on their 50 gram glucose test may have gestational diabetes during pregnancy, and require a follow-up test called a 3-hour oral glucose tolerance test (OGTT).

The OGTT requires dietary preparation. For three days prior to the test, 150 grams of carbohydrates should be consumed daily. Then a fasting period of 10 to 14 hours before the test is required; only water is allowed during this time. Because of the fasting component, the OGTT is usually performed first thing in the morning in a doctors office or outpatient lab. An initial blood sample is drawn to measure fasting blood glucose levels, and then the patient is given 100 grams of a glucose drink (the same type as that given in the glucose challenge) to consume. After that, blood samples are drawn every hour for 3 hours and blood glucose levels are measured.

The American Diabetes Association sets the following guidelines for results from the OGTT:


  • Fasting Blood Glucose Level95 mg/dl (5.33 mmol/l)
  • 1 Hour Blood Glucose Level140 mg/dl (7.8 mmol/l)
  • 2 Hour Blood Glucose Level120 mg/dl (6.7 mmol/l)

If two or more blood sugar levels are higher than the levels indicated, gestational diabetes is diagnosed.


1 - American Diabetes Association. ADA Standards of Medical Care in Diabetes?2010. Diabetes Care, Volume 33, Supplement 1, January 2010.

Reviewed by Francine Kaufman, MD. 4/10

Last Modified Date: May 24, 2013

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