Reporting Hemoglobin A1c: Do the Units Matter?

Abstract

Hemoglobin A1c (Hb A1c)2 is the analyte most commonly measured in clinical laboratories for patients with diabetes mellitus. Physicians use Hb A1c to monitor long-term glycemic control, adjust therapy, and predict complications of diabetes. Recently, Hb A1c was added as a criterion for the diagnosis of diabetes. Unlike most analytes, many patients with diabetes know their Hb A1c 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 chromatography) in 1958 (1) and was identified 8 years later as a Schiff base between hemoglobin and a ketone or aldehyde (2). Early commercial assays for Hb A1c were limited by the absence of standardization. For example, a comparison of 7 methods in 1992 revealed very large differences (>200%) in Hb A1c values among methods (3). Standardization programs were developed soon after 1993, when the Diabetes Control and Complications Trial (DCCT) documented the clinical value of Hb A1c in patients with type 1 diabetes (4). The publication 5 years later of results for the UK Prospective Diabetes Study (UKPDS), which revealed that decreasing Hb A1c in patients with type 2 diabetes reduced complications (5), further emphasized the essential contribution of Hb A1c monitoring to patient care. Importantly, the Hb A1c results obtained in the UKPDS were aligned with those in the DCCT. Therefore, appropriate patient management requires that an individual's Hb A1c value be traceable to DCCT/UKPDS values. The NGSP, previously known as the National Glycohemoglobin Standardization Program, was formed to achieve this objective. The NGSP laboratory network assists manufacturers of Hb A1c assays in calibrating their methods so that patient values correlate with those …