Improving glycemic control reduces the development of microvascular and macrovascular complications of diabetes mellitus. HbA1c measured by various methods is used to monitor glycemic status. In 1990s, various national standardization programs were implemented to reduce variability in HbA1c results. Furthermore, reference material and reference method were developed by IFCC to provide traceability. The results of the national programs were compared with the reference method and a linear regression curve was obtained. Based on these results a master equation was created. Nowadays, HbA1c is implemented in the diagnostic criteria of diabetes and many diabetes and clinical chemistry associations recommend measuring HbA1c with certified methods.