Glucose Tolerance Tests

Glucose Tolerance Tests Screen for Diabetes and Other Blood Sugar Disorders

Also known as: GTT, oral GTT (OGTT), blood sugar, blood glucose, fasting blood sugar

Your doctor may order a glucose tolerance test (GTT)  during pregnancy, or if you have symptoms suggesting diabetes, pre-diabetes or hypoglycemia, conditions that, left untreated, can result in severe effects including organ failure, brain damage, coma or even death. If you are diabetic, glucose tests (random or fasting) may be used frequently to monitor your blood sugar levels.

Results from the GTT are used to determine whether your blood glucose (sugar) level is within normal ranges. The test ordered may be one of four types:

  • Two-hour post meal test
  • Two-hour post glucola test
  • Three-hour test
  • Five-hour test

Preparation and Procedure

Two-Hour Post Meal Test

  • This test can be performed on a walk-in basis; you do not need to schedule an appointment.
  • You will need to eat a high-carbohydrate meal with large amounts of starches and sugars approximately two hours before the test. Your doctor will provide you with a menu of recommended foods and beverages.
  • Note the time you finish eating. You will need to have your blood drawn for the test two hours after your meal.
  • Arrive at the lab at least 20 minutes before the two hours is up.
  • Register and inform the clerk you have a timed test. A phlebotomist will draw blood from your arm at the appropriate time.

Two-Hour Post Glucola Test

  • Call the lab at least three days in advance to schedule an appointment.
  • For three days prior to your test, eat your normal diet but include an extra slice of bread at each of your three daily meals. If your doctor has given you a special diet for those three days, follow that instead.
  • For 10 or 12 hours before coming to the lab, be sure to follow your doctor’s fasting instructions. [link to fasting page] Failure to do so could lead to inaccurate test results.
  • Arrive at the lab and register for your appointment. Be sure to wear clothing with sleeves that are easily rolled up. A phlebotomist will draw blood from your arm for a “fasting glucose” specimen.
  • An analysis of your fasting specimen will be conducted before the test continues, and may take up to 30 minutes.
  • Once the analysis is complete, you will be given a sugar solution called glucola, which you will be asked to drink. Make sure you drink the entire amount within a five-minute period.
  • At the end of two hours, the phlebotomist will draw another specimen from your arm, which completes the two-hour glucola test.  During these two hours, you should remain in the lab and continue fasting. You may wish to bring reading material while you wait between blood draws.
  • Have something to eat as soon as possible after the completion of the test

Three- and Five-Hour Tests

The procedures for three- and five-hour tests are the same as for the two-hour post glucola test, except that your blood will be drawn at hourly intervals.

Time Required

Your total time in the lab will depend on the type of test:

  • Type of Glucose Tolerance Test Approx. Time Required:
    • Two-hour post meal 45 minutes
    • Two-hour post glucola 3 hours
    • Three-hour 4-1/2 hours
    • Five-hour 6 hours

Results from your test are typically sent to your doctor within days; be assured, however, that any critical results are communicated immediately. Your doctor will discuss your test results with you once he or she has reviewed them.


Michigan law requires that a valid order signed by an authorized person be presented before any laboratory test or procedure can be conducted. Authorized persons are defined as physicians, physician assistants, or nurse practitioners. These professionals are legally responsible for interpreting the results of tests based on their knowledge of the individual patient.

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