Haemoglobin A1c (HbA1c) reflects the glycaemic status of the latest 2-3 month and is used in both diagnosing and monitoring diabetes. Different circumstances may lead to spurious HbA1c results as summarised in this review. HbA1c is susceptible to changes in erythrocyte turnover (e.g. anaemia) regardless of measurement method, and to analytical interference (e.g. haemoglobin variants) depending on the method. The laboratory may detect and warn of suspected analytical interference. However, if the clinical presentation and glycaemic measures are incoherent, spurious HbA1c should be suspected and fasting glucose should be measured.