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We studied the magnitude, significance, and origin of an analytic bias that emerged between our point-of-care (POC) and our central laboratory (CL) methods for the measurement of hemoglobin A1c (HbA1c) and evaluated the analytic accuracy methods relative to the of 7 commonly used HbA1c National Glycohemoglobin Standardization Program (NGSP) reference method. The POC and CL methods were compared by split-sample analysis of clinical specimens and time series analyses of the HbA1c results reported for a 33-month period. The relative methods were evaluated using accuracies of 7 HbA1c College of American Pathologists proficiency survey results. Long-term drifts in the CL- and POC-analyzed test results caused the median intermethod bias [(POC result) – (CL result) ] to increase from –0.4% to –0.9% HbA1c. Systematic biases, drifts in analytic performance over time, and intermethod variability were frequently methods. observed among the 7 NGSP-certified HbA1c Intermethod variability is a potential source of results are interpreted inaccuracy whenever HbA1c relative to universal, fixed, clinical decision thresholds.