BACKGROUND:Monitoring of hemoglobin A(1c) (HbA(1c)) is important in the management of diabetes. The IFCC reference measurement procedure for HbA(1c) is based on the ratio of glycated to nonglycated N-terminal hexapeptides of the beta-chains of hemoglobin after digestion with Glu-C endoproteinase. We developed a modification of the original reference measurement procedure with HPLC-electrospray ionization/mass spectrometry (ESI/MS). METHOD:We performed chromatographic separation of the hexapeptides using a C12 reversed-phase column and a binary gradient system consisting of a mixture of H(2)O/acetonitrile/formic acid. RESULTS:Using this method, we obtained higher signal intensities and improved system stability compared with the reference measurement procedure. In the range of 3% to 14% HbA(1c), intralaboratory CVs were 0.71% to 1.86%. Deviations from IFCC target values were -0.87 to 1.00 relative %. These values fulfill acceptability criteria for HbA(1c) determination set by the IFCC Working Group on HbA(1c) Standardization. CONCLUSIONS:This procedure for the determination of HbA(1c) improves the existing reference measurement procedure.