Glucose Tolerance Test (Oral) for Diabetes and Gestational Diabetes (GTT)

Oral Glucose Tolerance Test for Diabetes and Gestational Diabetes
Tube Type(s)
Fluoride Oxalate (Grey Top)
Clinical Indication
To screen for gestational diabetes (GDM) at 24-28 weeks in maternity patients with indicative history. These include:
–        Previous GDM diagnosis
–        BMI ≥ 30 kg/m2
–        Previous macrosomic baby
–        First degree relative with diabetes
–        Family origin with raised prevalence of diabetes
May be used to diagnose diabetes mellitus in cases when reliability of glycated haemoglobin is in question e.g. anaemia
Target turnaround time
1 working day
Special requirements & notes
Patient is advised to fast for 10-12 hours prior to the test. Small quantities of water are permissible.
Prepare a solution of glucose to drink – adults are given a 75g anhydrous solution. A pre-prepared solution may be available from pharmacy for this purpose.
The patient should remain at rest throughout the duration of the test.
Collect a basal sample for glucose (grey top)
Ensure the patient drinks the prepared solution within 5 minutes.
Take a second blood sample (grey top) 120 minutes after the glucose drink has been consumed.
Send all samples together to the laboratory with one request form for the series. Please indicate the time of sampling on each bottle e.g. 0/120 minutes
Clinical Report
Diagnosis of Gestational Diabetes (GDM):
If fasting glucose is ≥5.6 mmol/L and/or 2 hour glucose is ≥ 7.8mmol/L, refer the patient to the Diabetes Specialist Nurse Team.
Diagnosis of Diabetes Mellitus (DM):
Fasting venous glucose >7.0 mmol/L (on more than one occasion in asymptomatic Patients) or a post 2 hour glucose load >11.1 mmol/L
Impaired Glucose Tolerance (IGT):
Fasting glucose of <7.0 and a post 2 hour glucose load between >7.8 and <11.1 mmol/L.
Impaired Fasting Glycaemia (IFG):
Fasting glucose is between >6.0 and <7.0 and a post 2 hour glucose load <7.8 mmol/L (if measured).
Volume required
2 samples (grey top) of 4ml each
If delayed transport to lab, store:
Factors affecting result interpretation
Patients are advised to refrain from smoking, eating, exercising or drinking (small amounts of water are permissible) during the test
Pathology discipline
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Posted in Assay.

Last updated: July 24, 2019