Diabetes mellitus is a chronic disease that is monitored by measurement of haemoglobin A1c (A1C) as an index of glycaemic control. The limitations of using A1C, given the consensus clinical practice recommendations made by the American Diabetes Association, need to be better understood by clinicians. These include bias between DCCT-aligned methods, analytical variation and intra-individual variation. As intra-individual variation is the principal factor determining variation in A1C in rolling means of the last four A1C results and to stable patients, clinicians may need to monitor A1C more frequently to achieve precise results. Laboratories need to report current values and the analyse six internal quality control specimens for each analytical run. 'Delta check' criteria ought to be applied and results reported to highlight acute deviations in A1C. Such procedures will aid the attainment of the clinical quality requirements and give appropriate results for audit purposes.