Glucose tolerance test

Most non-pregnant patients do not require a GTT for the diagnosis of diabetes mellitus. A GTT should not be performed if:

1. The patient is ill. The test is invalid in the presence of intercurrent illness (eg, infection), or

2. The patient has symptoms suggestive of Diabetes mellitus, with either fasting plasma glucose ≥ 7.0 mmol/L or random plasma glucose ≥ 11.1 mmol/L, or two results in these ranges in the absence of symptoms.

3. In patients after recent surgery or trauma, which may impair glucose tolerance.

4. The patient is known to have Diabetes mellitus.

5. The test may be invalid if the patient is taking corticosteroids or β adrenergic agonists - consult pathologist.

HbA1c has been approved for the diagnosis of Diabetes mellitus in Australasia, although this is not appropriate for children or pregnant women.

For three days prior to the test the patient must be on a diet containing approximately 150 g of carbohydrate daily, i.e. the patient should not be on a calorie restricted diet.

Drug therapy should be noted on the request form and reviewed with the pathologist.

The test is performed immediately following a fast of at least 8 h, but no more than 16 h.

A fasting blood glucose may be performed immediately prior to the GTT. If the level is > 7.0 mmol/L, the GTT is usually not performed, as this level is diagnostic of diabetes mellitus.

Dose of oral glucose: adult - 75 g; child - 1.75 g/kg body weight (75 g maximum), but recommend to discuss with pathologist before testing.

Reference intervals (non-pregnant):

Normal: Fasting < 6.1 mmol/L; 2 h < 7.8 mmol/L

Impaired glucose tolerance: Fasting < 7.0 mmol/L; 2 h 7.8-11.0 mmol/L

Impaired fasting glycaemia: Fasting 6.1-6.9 mmol/L; 2 h < 7.8 mmol/L

Diabetes mellitus: Fasting > 7.0 mmol/L; 2h > 11.1 mmol/L

Gestational diabetes:

Fasting ≥ 5.5 mmol/L and/or 2 h glucose ≥ 8.0 mmol/L

Note there are alternate diagnostic criteria.

International Association of the Diabetes and Pregnancy Study (IADPSG):

0 h ≥ 5.1 mmol/L

1 h ≥ 10.0 mmol/L

2 h ≥  8.5 mmol/L

However at this point there is no consensus on the diagnostic criteria.

Diagnosis of gestational Diabetes mellitus or suspected diabetes when the fasting glucose is 5.5-6.9 mmol/L or a random glucose is 7.8-11.0 mmol/L.  

In the presence of symptoms suggestive of Diabetes mellitus fulfilment of either criterion is sufficient to diagnose Diabetes mellitus.

In an asymptomatic patient fulfilling either criterion for Diabetes mellitus, at least one other diagnostic level on another occasion is required to establish the diagnosis of diabetes.

If there is still doubt the patient should be classified as having impaired glucose tolerance on the rationale that the final diagnosis of Diabetes mellitus should be unequivocal.

A pregnant patient should be regarded as having gestational diabetes if fasting glucose is > 5.5 mmol/L or 2 h glucose is > 8.0 mmol/L, although these criteria are not uniformly agreed.

If the 2 h level is < 11.1 mmol/L but is greater than the 1 h level the carbohydrate tolerance status of the patient cannot be determined as delayed absorption is present.