Longitudinal Sigma-Metric Profiling of Two Clinical Chemistry Platforms Using Peer-Group Internal Quality Control

Background: The Six Sigma metric benchmarks analytical performance, but most reports assess one platform at one time point, merging trueness with imprecision. Using routine peer-group internal quality control (IQC), we profiled two clinical chemistry platforms over five months. Because peer-group IQC compares imprecision directly but positions trueness only against same-method peers, we characterise rather than rank cross-platform trueness. Methods: Bio-Rad Unity peer-group IQC (January-May 2026) spanned two Siemens Dimension EXL analysers and two Roche cobas pro channels, two QC levels, and 30 analytes. TEa followed the CLIA 2024 Final Rule (25 analytes; provisional EFLM goals for five non-CLIA analytes). The Standard Deviation Index (SDI) referenced trueness to same-method peers, the CV ratio (CVR) imprecision, Sigma = (TEa - |bias|)/CV. Analyte-level comparison used Wilcoxon signed-rank testing, a confirmatory mixed-effects model, and Westgard SQC mapping. Results: Across 25 CLIA-regulated analytes, median Sigma was 5.9 (IQR 4.8-8.2) for cobas pro versus 4.0 (2.3-6.1) for Dimension EXL: a Hodges-Lehmann difference of 2.4 (95% CI 1.7-3.2; p < 0.001), confirmed by CLIA-only (2.51) and mixed-effects (2.7) analyses. Both aligned with their peers (SDI near zero); the gap reflected imprecision (median CV 1.7% vs. 2.5%; CVR 0.83 vs. 1.02), stable across months and instruments. The ranking reversed for iron, GGT and amylase; sodium, chloride, urea and creatinine were TEa-limited. The SQC redesign averaged N = 6 versus N = 3 controls/run (Dimension vs. cobas pro). Conclusions: A precision-driven difference separated the platforms as implemented here. Cross-platform trueness could not be ranked, yet decomposing Sigma localised precision limits, isolated calibration outliers, and produced analyte-specific SQC strategies needing prospective validation.