Improving Internal Quality Control Through Metrological Reinterpretation of Westgard Rules

Quality control (QC) in clinical laboratories is fundamentally influenced by metrological principles, yet several widely accepted assumptions underlying QC practices, especially the Westgard rules, remain insufficiently reexamined. This study aims to reassess the true efficiency of commonly applied Westgard rules by addressing inconsistencies between their theoretical derivation and practical implementation. Using analytical calculations of error detection probability, false rejection rates, probability of unnecessary rejection, and average run length, multiple QC rule sets were evaluated under different assumptions regarding standard deviation estimation, particularly the use of repeatability standard deviation (sr) versus within-laboratory reproducibility standard deviation (sRW). The results demonstrate that Westgard rules perform as expected only when charts are constructed using sr; however, this leads to excessive unnecessary alarms. The Westgard-recommended substitution with sRW introduces bias variability-dependent distortions, substantially reducing error detection efficiency as the sRW/sr ratio increases. On graphs designed with sr, the cross-run rules were found to contribute minimally to immediate error detection and are often overestimated in their effectiveness. Newly proposed rules based on sr and adjusted quality control decision limits, including a run average mean-based rule (Rra−3S), showed superior performance by balancing reliable error detection with reduced unnecessary rejections. These findings indicate that current Westgard-based QC systems rely on flawed assumptions, and a revised framework grounded in consistent metrological principles and stable parameters is necessary for more accurate and reliable QC in clinical laboratories.