Global Harmonization of Hemoglobin A1c

Measurement of glycohemoglobin (GHb) is widely used in patients with diabetes mellitus as a monitor of longterm glycemic control (1–3). In addition, prospective randomized clinical trials, most notably the Diabetes Control and Complications Trial (DCCT) and the United Kingdom Prospective Diabetes Study (UKPDS), have demonstrated that GHb is a measure of the risk for the development of diabetes complications (4, 5). GHb is therefore an integral component of the management of patients with diabetes. GHb comprises several different hemoglobin–glucose adducts, including hemoglobin A1a (HbA1a), HbA1b, and HbA1c. More than 30 different methods are commercially available to measure GHb. Together these factors have led to considerable variation in reference intervals and results reported by different laboratories. When the DCCT was published in 1993, the lack of standardization of GHb methods produced very wide variability among methods, with values ranging from 4.0% to 8.1% on the same blood sample (6 ). In the United States, the NGSP (previously known as the National Glycohemoglobin Standardization Program) has reduced interlaboratory variation (7 ). Using a standardization process based on the DCCT reference method, the NGSP has promoted a dramatic improvement in comparability of GHb values among laboratories (3 ). Data from the 2003 GH2 survey from the College of American Pathologists indicated that 98% of participating laboratories use NGSP-certified methods and report results as HbA1c or HbA1c equivalents (3 ). Analogous standardization programs in Sweden and Japan (8, 9), established to harmonize GHb results, have also reduced variability among GHb results. More recently, the IFCC Working Group on HbA1c Standardization prepared primary reference materials of pure HbA1c and HbA0 and developed a reference method for HbA1c (10 ). They defined HbA1c as the stable adduct of glucose to the N-terminal valine of the -chain of hemoglobin. In the reference method, hemoglobin is cleaved by endoproteinase Glu-C. The resulting glycated and nonglycated N-terminal hexapeptides are separated by HPLC, followed by quantification by electrospray ionization mass spectrometry or capillary electrophoresis (10 ). HbA1c is measured as the ratio of glycated to nonglycated N-terminal peptide and is reported as a percentage. Comparison of pooled blood samples revealed a linear relationship between HbA1c results of the IFCC reference method and the standardization schemes in the United States, Japan, and Sweden (11 ). For example, the calculated regression equation for NGSP