Glucose Tolerance Test

Last updated: 7 December 2017

What is a glucose tolerance test?

The glucose tolerance test (GTT) is a timed series of blood glucose measurements taken after a glucose load is taken by the patient.

People with normal glucose metabolism will have a relatively modest rise in their blood sugar levels, whereas those with diabetes or impaired glucose tolerance will have a higher peak of their blood glucose levels.

When would you need a glucose tolerance test?

A GTT may be ordered by your doctor if they are suspicious of diabetes or impaired glucose tolerance, a milder form of disease which may occur before full-blown diabetes develops.

A single fasting or random blood glucose test may show a glucose level high enough to diagnose diabetes, but if there is doubt a glucose tolerance test may be needed.

Some women develop impaired glucose tolerance or gestational diabetes during pregnancy only, and a GTT is one way to detect this.

Symptoms of diabetes include:

  • Excessive thirst leading to increased fluid intake (polydipsia)
  • Frequent urination or increased urine output (polyuria)
  • Weight loss or weight gain
  • Frequent infections (skin infection, thrush etc.)

How is a glucose tolerance test performed?

The GTT is a blood test performed in the morning, after an 8 to 16 hour overnight fasting period. During this fasting period (at home), patients are not allowed to eat or drink, smoke, or exercise excessively.

The test takes 2 hours and is performed at the laboratory’s collection centre. Blood tests are taken at the beginning of the test and at regular intervals after a glucose-containing drink is taken.

Sometimes it is possible to take the blood samples from a small plastic tube left in the vein – an intravenous cannula.

Glucose tolerance test results explained

The GTT result will be forwarded to your doctor after several days. It may show:

  • Normal test
  • Impaired glucose tolerance – this is not diabetes, but may progress to diabetes; it usually requires a modification to the diet
  • Diabetes mellitus – which may require diet control, oral medications, or insulin injections

Related specialists

Related procedures

  • Blood Test (venesection)
  • Intravenous cannulation
  • Pre-operative anaesthetic assessment

Related tests

Also known as

  • Oral glucose tolerance test 
  • OGTT
  • GTT



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This article is for informational purposes only and should not be taken as medical advice. If in doubt, HealthEngine recommends consulting with a registered health practitioner.