There is a need to simplify screening tests for type 2 diabetes mellitus (T2DM) so patients can be identified earlier and more efficiently. Glycated haemoglobin (HbA1c) has been recommended by some international organisations as a diagnostic tool for detecting T2DM and impaired glucose regulation (IGR, also termed prediabetes and includes impaired fasting glucose and/or impaired glucose tolerance). The HbA1c cut-point of ≥6.5% (48 mmol/mol) has been selected as diagnostic for T2DM, while the cut-points for IGR are debated by the different international organisations: an International Expert Committee has suggested using HbA1c 6.0-6.4% (42-46 mmol/mol); however, the American Diabetes Association has recommended using HbA1c 5.7-6.4% (39-46 mmol/mol). Some countries will adopt a new method of reporting HbA1c values in millimoles per mole (mmol/mol). Use of HbA1c has some logistical advantages over using an oral glucose tolerance test (OGTT). As patients do not need to fast, appointments do not need to be limited to the morning. The HbA1c result reflects longer term glycaemia and is less affected by recent physical/emotional stress. However, there is some debate as to whether HbA1c should replace fasting plasma glucose or the OGTT. As the two tests detect different people, some individuals with diabetes detected on OGTT will no longer be classified as having T2DM using HbA1c ≥6.5% criteria. Furthermore, some medical conditions can result in HbA1c assay measurements not reflecting glycaemic control over the last 2-3 months; these include haematological disorders, renal failure, and chronic excess alcohol consumption.