Laboratory Test Reference Guide

 For three days prior to the test, the patient must be on a diet containing no less than 150 g of carbohydrate daily. Do not perform this test on an acutely unwell patient or known to be diabetic. The patient must have fasted for at least 8 hours. GTT should be performed in the morning because fasting glucose levels decrease during the day. Laboratory Phlebotomists carry out this test. Bookings for tests at Waikato Hospital should be made in advance by paging Phlebotomists directly.

GTT's are rarely performed. HbA1c is the preferred test when screening for diabetes . A fasting glucose should be used when HbA1c is unreliable, for example when there is increased red cell turnover (e.g. haemolytic conditions, recent or ongoing blood loss). The patient must be at rest and not smoke during the procedure.  The glucose polymer solution for this test should be obtained from the pharmacy if not available in your area. Following an overnight fast, an oral dose of glucose polymer is given. Adults drink the whole bottle ( = 75 g glucose). Children weighing less than 43 kg take the amount of solution equal to 1.7 g/kg.

Diagnosis of Diabetes Mellitus

If symptomatic: Fasting glucose >6.9mmol/L  AND/OR 2hr value >11.1 on ONE occasion.

If asymptomatic: Fasting glucose >6.9mmol/L AND/OR 2hr value >11.1 on TWO  occasions.

Impaired fasting Glycaemia:

Fasting glucose between 6.1 and 6.9 mmol/L and (if measured) 2 hr glucose < 7.8 mmol/L

Impaired glucose tolerance:

Fasting glucose < 7 mmol/L and 2 hr glucose  between 7.8 mmol/L and 11 mmol/L