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.