Numerical and Interpretive Comparability of Optical and Mechanical Coagulation Analyzers: A Zlog-Supported Method Comparison Study

Background/Objectives: In laboratories using coagulation analyzers with different clot-detection principles and platform-specific reference intervals (RIs), numerical agreement does not necessarily ensure concordant clinical interpretation. We evaluated how analytical agreement and RI-based interpretation diverge between mechanical and optical coagulation analyzers. Methods: Paired duplicate measurements were performed on the STA R Max (Stago) and ACL TOP 750 LAS (Werfen) for prothrombin time (PT), activated partial thromboplastin time (APTT), thrombin time (TT), fibrinogen (FIB), international normalized ratio (INR), and antithrombin (AT). Numerical agreement was assessed by Passing–Bablok regression and log-ratio Bland–Altman analysis against predefined total allowable error (TEa) limits. Each result was standardized using its platform-specific RI, and the between-platform zlog difference (Δzlog) described differences in RI-relative position. Results: Stago yielded higher values than Werfen for PT, APTT, TT, and FIB, with mean biases of +15.6% to +20.4%, whereas INR and AT showed close agreement (−1.1% and +1.6%). Interpretive concordance among the primary assays ranged from 61.9% for APTT to 89.2% for FIB. The analytical-by-interpretive matrix identified specimens that were within TEa but classified differently and specimens that were beyond TEa but retained the same RI classification. Median Δzlog values were −0.43 for PT, +0.09 for APTT, −0.66 for TT, and +0.77 for FIB. Conclusions: Numerical and interpretive comparability are distinct dimensions. Combining the analytical-by-interpretive matrix with Δzlog can reveal clinically relevant divergence not apparent from bias-versus-TEa assessment alone. In this purposively selected dataset, divergence arose mainly from RI offset for TT and analytical dispersion for APTT; the sample-specific discordance rates are not population prevalence estimates.