Oral glucose tolerance test

Indications

Diagnosis of diabetes mellitus and impaired glucose tolerance.

Patient preparation

The patient should be aware of the following:

They should be on a normal carbohydrate containing diet (at least 150 g/day) for at least three days before the test.

They should fast from 10pm on the evening before the test with the exception of water and any drugs normally taken.

Procedure

At about 9am, take 2 mL venous blood into a fluoride oxalate (grey top). Give the patient 75 g glucose.

This can be given either as: 394 mL of Lucozade Energy original (73 kcal/100 mL formulation) consumed over a 5 minute period; 75 g of anhydrous glucose dissolved in ~250 ml of water; or, take one further venous blood sample into a fluoride oxalate bottle 120 minutes after the glucose was given.

Side effects

The test is usually well tolerated.

Rarely, some patients may feel nauseated and experience vaso-vagal symptoms during the test. The test is invalidated by vomiting.

Interpretation

Results are interpreted in accordance with WHO criteria. These are as follows (for non pregnant people):

Classification

Fasting

Two hours

Normal

<6.1

<7.8

Impaired fasting glucose

≥6.1 and <7.0

<7.8

Impaired glucose tolerance

<7.0

7.8 and <11.0

Diabetes mellitus

≥7.0

≥11.1

 

Venous plasma glucose concentrations in mmol/L.

The test should not be performed in individuals who are acutely unwell or who are within six weeks of an MI or major surgery.

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