Residual Safety Margin-Based Risk Stratification for Hospital-Wide POCT Glucose Meters Anchored to ISO 15197: Moving Beyond Pass-Fail

Background: In this hospital-wide evaluation of point-of-care testing (POCT) glucose meters, we introduced a residual safety margin (r) anchored to ISO 15197:2013 thresholds to quantify tolerance, move beyond binary pass/fail assessments, and enable risk stratification. Methods: Thirty-five departmental glucose meters were compared with a central laboratory reference at five predefined glucose concentrations. Compliance was assessed using ISO 15197:2013 point-wise limits, Bland–Altman analysis was used to estimate bias and limits of agreement, and the mean absolute relative difference (MARD) and root mean square error (RMSE) were calculated to summarize overall error. At each concentration, r was calculated for every department, ranked, and classified into low, medium, or high risk using allowable error thresholds based on biological variation, specifically total allowable error (TEa), mapped to the ISO limits. Results: All departments met ISO criteria (100% compliance; 95% CI: 97.9–100%). Mean bias was −1.43 mg/dL, with limits of agreement from −15.6 to 12.8 mg/dL; MARD was 3.8% (95% CI: 3.4–4.3%), and RMSE was 7.4 mg/dL (95% CI: 6.6–8.2 mg/dL). Despite universal compliance, r-based analysis revealed concentration-related heterogeneity and highlighted borderline-performing departments that were overlooked by conventional metrics. Conclusions: By anchoring residual safety margins to ISO thresholds, the r framework shifts POCT glucose assessment from a binary pass/fail decision to a risk-stratified ranking approach, exposing latent performance variation and supporting targeted quality improvement at the hospital level.