Glucose Tolerance Test

At Central Vermont Medical Center's Outpatient Laboratory we conduct oral glucose tolerance tests to to diagnose Diabetes Mellitus.

The oral glucose tolerance test (GTT) is the most widely used test to diagnose Diabetes Mellitus.  It is a lengthier test, usually performed to confirm the results of a prior screening test, such as a one hour PC. It looks at the way your body processes blood sugar or glucose.   

How Long Will It Take?

Your doctor may order a two or three hour glucose tolerance test, depending on your symptoms or history.  To allow ample time for your test, you should be prepared to be at the hospital for:

  • up to 3 hours for the 2 hour GTT
  • up to 4 hours for the 3 hour GTT

Patient Preparation

  • For 3 days prior to the test, you should be on an unrestricted diet containing at least 150 grams of carbohydrate per day. The average diet should meet these carbohydrate requirements.
  • You should remain physically active in the days prior to the test. Hospitalized, acutely ill or inactive patients should not be tested.
  • For 3 days prior to the test, you should discontinue medications known to affect glucose utilization, such as salicylates, diuretics, anticonvulsants, propranalol, estrogens, birth control pills and glucocorticoids. Check with your doctor prior to discontinuing any medication.   
  • You should fast (nothing to eat or drink, with the exception of water) for 8-12 hours prior to the test. Avoid black coffee and smoking.  
  • Do not exercise for 8-12 hours prior to the test.  
  • Feel free to bring a book, or something else to keep you occupied during the test.

How the Test is Performed

Upon arrival at the hospital, you will need to check in at Registration. Once you have registered, you will be directed towards the Outpatient Lab.

  • At the beginning of your test, you will have a fasting blood draw.  
  • You will be asked to drink an orange-flavored beverage as quickly as possible.  
  • After you have finished the drink, you will be drawn at 60 minute intervals, depending on which test your doctor has ordered.  You will have a total of:
    • 2 blood draws for the 2 hour GTT
    • 4 blood draws for the 3 hour GTT

During your test, you will be asked:

  • To remain on hospital grounds.
  • To remain inactive.
  • To not smoke or chew gum.
  • To not have anything to eat or drink, however you may have a few small sips of water.
  • To inform lab personnel if you are feeling ill or vomit at any time during the test.

At the End of Your Test

  • After your last blood draw, for patient safety purposes, you will be asked to wait while we process your last specimen STAT.  We want to be sure your blood sugar level is acceptable before we allow you to leave. Abnormal results may need to be reviewed by a Pathologist. Please understand that this last critical step may take up to 30 minutes.
  • While you wait, we will offer you some juice and crackers.
  • Once the lab has had the opportunity to review your result, if you are feeling fine, you are free to go.
  • You should be receiving the results from your doctor within two weeks of test performance.
Central Vermont Medical Center Campus

CVMC Laboratory

Central Vermont Medical Center
130 Fisher Road
Berlin, VT 05602

Please check in at Registration in the Hospital's main lobby




6:30 am-5:00 pm
6:30 am-5:00 pm
6:30 am-5:00 pm
6:30 am-5:00 pm
6:30 am-5:00 pm
8:00 am-2:00 pm

Please check in at registration in the hospital's main lobby.