Before the Glucose Tests

  • Report to your doctor any medications, herbs, or supplements you are taking. You may be advised to discontinue certain of these agents before all of these tests, except HbA1c.

Fasting glucose test:

  • Do not eat or drink anything for 8 hours before the test.

Postprandial glucose test:

  • A fasting blood glucose test is performed to establish the pre-test blood glucose level.
  • You will be instructed to eat a balanced meal containing at least 75 grams of carbohydrates, and then fast for 2 hours.
  • Do not smoke or perform any strenuous activities after the meal.

Oral glucose tolerance test:

  • Your doctor will advise you to maintain a high-carbohydrate diet for 3 days before the test.
  • You should fast for 8 to 10 hours before the test. Water is permitted.
  • Do not smoke, exercise strenuously, or drink coffee or alcohol for 8 hours before the test.
  • Your doctor may weigh you to determine the appropriate amount of glucose to include in the test beverage.
  • You may want to bring something to read during the test.

HbA1c test:

  • No advance preparation is necessary.

What You Experience during Glucose Tests

Fasting, postprandial, and HbA1c tests:

  • A sample of your blood is drawn from a vein, usually in your arm, and sent to a laboratory for analysis.

Oral glucose tolerance test:

  • Blood will be drawn using venipuncture and a urine specimen will be obtained to determine fasting glucose levels.
  • You will be given a glucose-laden beverage. It will be very sweet. (It may be diluted with a small amount of lemon juice and water.)
  • You won’t be allowed to eat anything until the test is complete, but you may drink water.
  • Additional blood samples are taken at 30 minutes, 60 minutes, and hourly intervals after you drink the glucose beverage.
  • Urine specimens are taken at hourly intervals.
  • The total test time is usually 3 hours, but may last up to 6 hours.


The Johns Hopkins Consumer Guide to Medical Tests

Simeon Margolis, M.D., Ph.D., Medical Editor

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 13 Jan 2012

Last Modified: 23 Jul 2013