Sigma metric and Quality Goal Index assessment of hemoglobin A1c analytical performance in an ambulatory accredited laboratory.

Abstract

Introduction: Hemoglobin A1c has a stringent total allowable error (TEa), making high Six Sigma values difficult to attain. In addition, data from ambulatory accredited laboratories are sparse.

Methods: Internal quality control (QC) data for 2 levels were collected over 8 months on a single-lot Bio-Rad D-10 high-performance liquid chromatography analyzer. Long-term imprecision was the coefficient of variation (s/x¯) × 100 (CV) of all internal QC results combined, capturing between-month variation. Bias was derived from external quality assessment (EQAS) and assigned QC targets, applied as absolute values. The Sigma metric was (TEa ‒ absolute bias) / CV at TEa levels of 6%, 7%, and 8%; the Quality Goal Index distinguished imprecision-driven from bias-driven performance.

Results: Long-term CVs were 3.04% (level 1) and 1.34% (level 2); EQASs were satisfactory throughout. At a TEa of 6% with EQAS-derived bias, Sigma metrics were 1.19 and 2.70 and, with a level-specific QC target bias, 1.46 and 4.22. At the Clinical Laboratory Improvement Amendments 2025 criterion of 8%, the Sigma metrics were 2.11 and 5.71. The level 1 Quality Goal Index (0.52) identified imprecision as predominant.

Discussion: Despite satisfactory external proficiency, Sigma metrics fell below 3 at both levels against a 6% TEa and below 2 at the clinically pivotal lower concentration, driven by imprecision rather than bias. No control strategy compensates for this; therefore, we proposed a precision-focused corrective plan that targets between-month instability.