How are prediabetes and diabetes diagnosed?
To diagnose diabetes or prediabetes, your doctor can choose from the following tests to measure the amount of glucose in your blood:
Fasting plasma glucose: a blood test to measure your blood glucose after 8 hours of fasting.
Oral glucose tolerance test: measuring blood glucose while fasted and 2 hours after taking 75 g of glucose.
Haemoglobin A1c: a measure of the amount of glycated haemoglobin in your blood. Haemoglobin A1c indicates how much glucose you have had in your blood in the preceding 3 months.
For these blood glucose tests, your results may fall in the ‘normal’, ‘prediabetes’ or ‘diabetes’ ranges as shown:
For fasting plasma glucose,
glucose equal to or less than 6.0 mmol/L or less than 110 mg/dL is normal;
glucose range between 6.1 mmol/L and 6.9 mmol/L or between 110 mg/dL and less than 126 mg/dL is considered prediabetic;
glucose equal to or greater than 7 mmol/L or 126 mg/dL on two separate tests indicates you have diabetes.
For 2-hour plasma glucose after an oral glucose tolerance test,
glucose less than 7.8 mmol/L or 140 mg/dL is normal;
glucose reading between 7.8 mmol/L and 11.0 mmol/L or between 140 mg/dL and less than 200 mg/dL indicates prediabetes;
glucose equal to or greater than 11.1 mmol/L or 200 mg/dL indicates that you have diabetes.
For Haemoglobin A1c,
a range between 4% and 5.6% is normal;
a range between 5.7% and 6.5% is considered prediabetic;
a value higher than 6.5% means that you have diabetes.
Twigg, S. M., Kamp, M. C., Davis, T. M., Neylon, E. K., Flack, J. R., Australian Diabetes Society, & Australian Diabetes Educators Association (2007). Prediabetes: a position statement from the Australian Diabetes Society and Australian Diabetes Educators Association. The Medical journal of Australia, 186(9), 461–465. https://doi.org/10.5694/j.1326-5377.2007.tb00998.x
Understanding A1C: Diagnosis from American Diabetes Association, 2021, https://www.diabetes.org/a1c/diagnosis
The Royal Australian College of General Practitioners. Management of type 2 diabetes: A handbook for general practice. East Melbourne, Vic: RACGP, 2020.