Glucose tolerance test

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The glucose tolerance test (GTT) is a diagnostic laboratory test to measure how the body deals with glucose (blood sugar). The glucose is usually given by mouth, but can in rare cases be given via drip.

The oral GTT is regarded as the "gold standard" in the laboratory diagnosis of diabetes.

Why is the test done?

Normally, glucose in the blood causes the pancreas to produce the hormone Insulin, which helps to get the glucose inside cells, where it can be used for energy. The amount, timing and effect of the insulin secretion are all geared to produce a smooth response, gradually clearing glucose from the blood.

In diabetes mellitus (DM), this response pattern is abnormal: glucose is not efficiently cleared from the blood, and the resulting high levels cause other problems. Abnormalities in the response pattern may be detected long before obvious DM is diagnosed: appropriate steps can thus be taken to manage the pre-diabetic state, and defer, or even avoid, the onset of full-blown diabetes.

Some other conditions, notably metabolic syndrome, also show abnormal GTT. Pregnancy-related diabetes may also give an abnormal response, but this is a specialised subject and is not part of this information sheet.

The GTT is thus purely for diagnosis. Other types of tests are used for monitoring progress or treatment, like fasting or random BG, or HbA1C measurement.

How is the test done?

The patient is required to fast for at least 8 hours before the test. The basic steps of the test are:

  • Set up an intravenous line for repeat blood sampling,
  • Take a blood sample for a fasting blood glucose measurement - this gives the baseline value,
  • Administer a standard (usually 75g) dose of glucose, taken by mouth,
  • Blood samples for glucose measurement are taken at fixed intervals (for instance, every 30 minutes), and
  • Blood Insulin may also be measured in some cases.

No special precautions or restrictions apply after the test.

Interpretation of results

For the diagnosis of DM, the fasting and 2-hour glucose values form part of the internationally accepted criteria. Slightly raised glucose levels indicate either impaired fasting glycaemia or impaired glucose tolerance (a pre-diabetic state). Markedly raised levels are diagnostic of DM.

Plasma glucose levels

Normal

Impaired fasting glycaemia

Impaired glucose tolerance

Diabetes mellitus

mmol/l

Fasting

2hrs

Fasting

2hrs

Fasting

2hrs

Fasting

2hrs

<6.1

<7.8

6.1-7.0

<7.8

<7.0

=7.8

=7.0

=11

A 3-hour test can be done, but is unnecessary, as the 2-hour results are reliable for the purposes of diagnosing DM.

It is important to document any medication used by the patient, as some may interfere with glucose metabolism, for example:

  • Some diuretics,
  • Beta blockers,
  • Oral contraceptives,
  • Steroids ( like prednisone), and
  • Some antidepressants and anti-convulsants.

Recent surgery or infection may also affect the results.

Risks

The only risks concern the taking of blood samples, and include

  • Bruising,
  • Bleeding from the puncture site,
  • Haematoma - a lump due to bleeding under the skin, and
  • Infection - always higher risk in DM.

(Dr A G Hall)

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