Glucose tolerance test

Friday, 04 June 2021

Glucose tolerance test

The Glucose tolerance test (GTT) measures how quickly a dose of glucose is cleared from the blood.

It is performed in the majority of our collection centres. Where possible, an appointment should be made at a collection centre with an internal toilet as you are required to remain in the centre for minimum of 2 hours. You may wish to bring a book or magazine to read while you are waiting.

NOTE: Please tell your collector if you have had bariatric surgery (e.g. bypass/gastric banding). GTT’s must commence prior to 10 am, unless confirmed with a pathologist prior to the collection.

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Postpone your test if you have:

  • Eaten in the 8 hours before the test, or fasted for more than 16 hours
  • Recently experienced a period of acute illness or prolonged bed-rest. (In this case, postpone your test for 2 weeks.
  • Suddenly developed a cold, the flu, or gastroenteritis



  • Before taking this test, you MUST fast i.e. not eat or drink anything except water for 8-16 hours before your blood test (12 hours optimal). On the day before your test, eat your normal evening meal, then DO NOT eat or drink anything (except water) after midnight. Continue fasting on the morning of your test (you may drink water, but no more than 2 glasses).
  • It is recommended that you adhere to your current diet in the days prior to testing.
  • If you are on a low carbohydrate diet it is best for you to discuss the impact of your diet with your medical practitioner.
  • For 8 hours before, and during, the test, avoid smoking and heavy exercise.


Continue any medications you take on a regular basis.

However, please inform the collection staff if you are taking any cortisone type medications (e.g. Prednisone) on the morning of your test. Please inform the collector if you are on a temporary course of medication. Some medications can affect the test results and the collector may advise you to postpone your test or seek your doctor’s advice.

Item number 34159 - Issue date August 2020