HbA1c: chasing numbers or considering context?

Glycated haemoglobin (HbA_1c) reflects the average blood glucose level in the three months preceding the test. Changes in consecutive HbA_1c tests indicate deteriorating, or improved, glycaemic control. HbA_1c is considered to be the "gold standard" measure of blood glucose control and is often used as the basis for prescribing choices and other care decisions. A number of factors can affect the accuracy of the HbA_1c result, for example, the life span of red blood cells, assay methods and clinicians' awareness of possible interfering factors. The aim of this article is to outline how HbA_1c is used as a diagnostic test, how it is used to monitor glycaemic control and how it can guide management decisions. It is also important to emphasise the importance of considering HbA_1c in the context of the individual rather than as an isolated number.