Pathology Handbook

Glucose Tolerance Test

Synonyms

GTT

Clinical Indications

To aid in diagnosis of diabetes mellitus.  The test should ONLY be used to help diagnose diabetes in patients with equivocal or borderline fasting glucose levels.

The test should NOT be carried out on patients with clearly abnormal glucose results; patients with clear symptoms of diabetes should have a fasting glucose level measured and a GTT should ONLY be requested if this is equivocal.

Test Includes

Fasting glucose and a second sample collected 2 hours after a 75g glucose load

Request Form 

Request on ICE

Availability

On request; patients must make an appointment with the laboratory for this test

Specific Criteria

Previous borderline or equivocal glucose results

Patient Preparation

Patient must fast overnight (10 hours) and be advised that they may only drink plain water during this period.  Medication should be taken normally.

Turnaround Time

Same Day (Monday to Friday)

Specimen

Plasma sample collected into fluoride-oxalate preservative

Volume

2 ml

Container

Vacutainer grey top

Collection

WHO protocol must be followed:

Patients must have 'normal' carbohydrate intake for 3 days before the test; restricting carbohydrate intake may give a falsely diabetic picture

Patients must have normal levels of exercise for the 3 days preceding the test [the test should be avoided in hospital in-patients on poor diet and having limited exercise]

Patients must fast correctly before the test

A fasting glucose sample is collected

75g dose of glucose is given as a drink, which must be consumed within 5 minutes

A second glucose sample is collected 2 hrs later

The patient must sit quietly during this 2 hours, resting. They must not smoke or eat during this time but may drink plain water.

WHO definition and diagnosis of diabetes

Causes for Rejection

Unlabelled sample
Not meeting specific criteria for analysis.  

Interpretation

Normal: Fasting glucose ≤6.0 mmol/L and 2 hr glucose <7.8 mmol/L

Diabetes: Fasting glucose ≥7 mmol/L and/or 2 hr glucose ≥11.1

Impaired glucose tolerance: Fasting glucose <7 mmol/L and/or 2 hr glucose ≥7.8 mmol/L and <11.1 mmol/L

Impaired fasting glycaemia: Fasting glucose ≥6.1 mmol/L and 2 hr glucose ≤7.8 mmol/L.  Patients with impaired fasting glycaemia have frequently fasted incorrectly and a repeat fasting glucose should be measured after emphasising the need to fast correctly for the test.