Glycated haemoglobin – HbA1C, or A1C as it is now being called – should be used to diagnose diabetes, according to a recent recommendation from an international expert committee appointed by the American Diabetes Association, the International Diabetes Federation and the European Association for the Study of Diabetes. We explore the basis for this recommendation, and look at why HbA1C would provide a more accurate measure for diagnosing diabetes than glucose tests and what the change would mean in clinical practice.