Reporting Hemoglobin A1c: Do the Units Matter?

2 is the analyte most com-monly measured in clinical laboratories for patients with diabetes mellitus. Physicians use Hb A1c to mon-itor long-term glycemic control, adjust therapy, and predict complications of diabetes. Recently,HbA1cwas added as a criterion for the diagnosis of diabetes. Un-like most analytes, many patients with diabetes know theirHbA1c value and are familiar with the target value they need to attain for good glycemic control. Glycation of proteins was recognized over 100 years ago. Hb A1c was first isolated (by chromatogra-phy) in 1958 (1) and was identified 8 years later as a Schiff base between hemoglobin and a ketone or alde-hyde (2). Early commercial assays forHbA1c were lim-ited by the absence of standardization. For example, a comparison of 7 methods in 1992 revealed very large differences (200%) inHb A1c values amongmethods (3). Standardization programs were developed soon thereafter (in 1993), when the Diabetes Control and Complications Trial (DCCT) documented the clinical values of Hb A1c for patients with type 1 diabetes (4). The publication 5 years later of results for the UK Pro-spective Diabetes Study (UKPDS), which revealed that decreasing Hb A1c in patients with type 2 diabetes re-duced complications (5), further emphasized the es-sential contribution of Hb A1c monitoring to patient care. Importantly, the Hb A1c results obtained in the UKPDSwere in accordwith those in theDCCT. There-fore, appropriate patient management requires that an individual’s Hb A1c value be traceable to DCCT/ UKPDS values