The Glucose Tolerance Test (GTT), also referred to as the Oral Glucose Tolerance Test (OGTT), is a method which can help to diagnose instances of diabetes mellitus or insulin resistance

The test is a more substantial indicator of diabetes than finger prick testing.

What is an OGT test?

The test is used to determine whether the body has difficulty metabolising intake of sugar/carbohydrate

The patient is asked to take a glucose drink and their blood glucose level is measured before and at intervals after the sugary drink is taken.

Why is an oral glucose tolerance test done?

This can be a useful test in helping to diagnose:

How is the test performed?

Before the test you will be asked not to eat, or drink certain fluids, for up to 8 to 12 hours before the test.

You may be asked to not take certain medications in the lead up to the test, but only if these would affect the test results.

For the test itself, you will first have blood taken to measure your blood glucose level before the test. The next stage is to take a very sweet tasting, glucose drink.

Further blood samples will then be taken either at regular intervals of say 30 or 60 minutes or a single test after 2 hours. The test could take up to 3 hours.

Between blood tests you will need to wait so it’s best to have some reading material, or something else to keep you occupied, with you.

What should the OGTT results be?

People without diabetes

  • Fasting value (before test): under 6 mmol/L
  • At 2 hours: under 7.8 mmol/L

People with impaired glucose tolerance (IGT)

  • Fasting value (before test): 6.0 to 7.0 mmol/L
  • At 2 hours: 7.9 to 11.0 mmol/L

Diabetic levels

  • Fasting value (before test): over 7.0 mmol/L
  • At 2 hours: over 11.0 mmol/L

What do these OGT test results mean?

If you are within the impaired glucose tolerance range, you will likely be advised to make lifestyle changes. In some cases, blood glucose lowering medication may be advised.

If you fall within the diabetic range, it is quite likely that blood glucose medication will be prescribed to help your body keep your blood glucose levels down.

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