Evaluation of HbA1c analytical performance using sigma metrics and total allowable error models

INTRODUCTION: Accurate measurement of glycosylated hemoglobin (HbA1c) is essential for the correct diagnosis and treatment of diabetes. The Implementation Task Force of the International Federation of Clinical Chemistry and Labora-tory Medicine (IFCC-TF) recommends the standardization of HbA1c testing and its monitoring using the sigma metric model. The aim of this study was to evaluate the analytical performance of the HbA1c test using sigma metrics accord-ing to different quality specifications and to examine its impact on quality control planning.
METHODS: Coefficients of variation were evaluated using IQC materials at two levels. Bias values were calculated from 12 months of PT results. Sigma metrics were calculated according to the Clinical Laboratory Improvement Amend-ments (CLIA) 2024, National Glycohemoglobin Standardization Program (NGSP), and IFCC total allowable error (TEa) criteria. Total analytical error (TAE) was also calculated and compared with TEa limits. The Quality Goal Index (QGI) was calculated from sigma values to identify the primary source of analytical error.
RESULTS: All CV values were below 3%, except for the 12-month pooled CV1 value. The weighted mean signed bias was −0.98%, while the weighted mean absolute bias was 1.99%. Sigma metrics for the 12-month period at IQC-1 were 2.18 (IFCC), 1.64 (CLIA), and 1.09 (NGSP), indicating acceptable performance only under the IFCC criteria. For IQC-2, sigma values were 4.05 (IFCC), 3.04 (CLIA), and 2.03 (NGSP), indicating acceptable performance under all three specifications.
DISCUSSION AND CONCLUSION: The performance of the same analytical system was classified differently depending on the selected quality specification. Long-term sigma results at low concentration levels near clinical decision limits were borderline according to the IFCC criteria and unacceptable according to the CLIA and NGSP criteria, suggesting that stricter quality control strategies may be required to ensure the reliability of patient results.

Keywords: Analytical performance, HbA1c standardization, sigma-metric