July 14, 2020
Oral Glucose Tolerance Test for the Diagnosis of Diabetes Mellitus Minimize

Oral glucose tolerance test for the diagnosis of Diabetes Mellitus


The diagnosis of diabetes is made on the basis of repeatedly elevated fasting plasma glucose. The use of the oral glucose tolerance test is to clarify borderline elevations in fasting plasma glucose. This test may be replaced by the use of glycated haemoglobin (HbA1c) in 2011.


This test is only necessary if fasting and/or random glucose measurements are equivocal ie 5.6 to 7.0 mmol/L.

This test should NOT be performed in patients who fulfil the criteria for diabetes mellitus. These are 1) a fasting plasma glucose >7.0 mmol/L or two or more occasions or 2) clinical symptoms of diabetes eg polydipsia, polyuria, ketonuria and rapid weight loss with a random plasma glucose of >11.1 mmol/L).

This test should not be performed in patients who are under physical stress eg post surgery, trauma or infection or extreme psychological stress as these may give misleading results.

This test should not be performed in patients with periodic hypokalaemic paralysis.


In normal individuals pancreatic insulin secretion maintains blood glucose within a tight concentration range following an oral glucose load. Failure of insulin secretion, or resistance to insulin action, will result in an elevation in blood glucose.

Side effects

Some subjects feel nauseated and may have vasovagal symptoms during this test.


Patients should be advised to eat a normal carbohydrate diet (>150g daily) for at least 3 days prior to the test and undertake normal physical activity.

Patients must fast for 10-14 hours prior to this test but may drink small volumes of plain water.

Smoking and physical exercise should NOT be allowed in the morning prior to, and during, the test.


Adults: 75 g anhydrous glucose in cold water. The solution should be chilled to improve palatability. An alternative is this Polycal ® (113 mL) which is more palatable and should be followed by 150mL water (total volume should be 250-300mL).

Children: the dose is weight related 1.75g/kg body weight: the maximum load is 75g.
Polycal ® contains 0.66g anhydrous glucose per mL (or 1.51mL contains 1g anhydrous glucose).


This test should be performed in the morning. Patients should remain at rest during the test.

There is no need to take urine samples for glucose measurements.

 Time 0 min

 1-2 mL blood should be taken in a fluoride oxalate tube
   The glucose solution or Polycal ® should be drunk over 5 minutes.
 Time 120 min  1-2 mL blood should be taken in a fluoride oxalate tube



Plasma glucose (mmol/L)


0 min

120 min

Non diabetic

< 6.0

< 7.8

Impaired glucose tolerance

6.1 - 6.9

7.9 - 11.0


> 7.0

> 11.1

JHB 22 Sept 2011


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