Conventionally diabetes mellitus has been diagnosed by high fasting or random blood glucose concentrations, or abnormal oral glucose tolerance tests (OGTT). Haemoglobin A1c (HbA1c) has been used to monitor longer term glycaemic control in patients with known diabetes mellitus.
In 2011, the World Health Organisation (WHO 2011) recommended that HbA1c measurements should also be used to diagnose diabetes in the majority of asymptomatic individuals, and this recommendation has been agreed in the UK (NHS Diabetes 2011).
HbA1c of 48 mmol/mol or more (≥ 6.5%) is consistent with diabetes: if the patient has no symptoms then a second HbA1c result must be obtained within 2 weeks, and if ≥ 48 mmol/mol (≥ 6.5%) confirms diabetes mellitus.
HbA1c values of 42 to 47 mmol/mol (6.0 to 6.4%) suggest a high risk of future diabetes. Such individuals should be offered structured lifestyle education and support to delay/prevent development of diabetes, and have an annual HbA1c test