Systematic overestimation of mortality by SAPS II in contemporary ICU practice: a retrospective study analyzing ICU patient outcome using the 2013-2024 French National Health Database.

BACKGROUND: The present study re-evaluates the performance of the Simplified Acute Physiology Score (SAPS) II to predict in-hospital mortality of Intensive Care Unit (ICU) patients by analyzing the French nationwide hospital stay billing database between 2013 and 2024. METHODS: From 2,702,204 adult ICU admissions, 80% (n = 2,161,763; (mean SAPS II = 41 ± 21.3) were used to fit logistic regressions of the in-hospital mortality probability as polynomial functions of the SAPS II up to the 5th degree. The remaining 20% (n = 540,441; (mean SAPS II = 41 ± 21.3)) of the analytical sample was used as the validation set on which we computed overall and age-stratified Brier scores for model selection and calibration comparison against the original SAPS II mortality prediction equation. Discrimination was checked using the receiver operating characteristic (ROC) curve. FINDINGS: The observed mortality rate in the validation set was 20.0%, while the original SAPS II mortality mapping predicted 32.0%. By minimizing the Brier score, we identify a quintic polynomial model that predicts an overall 20.0% mortality rate. This new calibration relatively improves by more than 19% the overall Brier score, and by more than 29% for intermediate SAPS II values (60-79). The discrimination remains similar (area under the ROC curve = 0.830 and 0.832 for the original SAPS II and quintic model, respectively). INTERPRETATION: Even if SAPS II maintains adequate discrimination, its systematic overprediction undermines its utility for contemporary critical care practice, suggesting a regular refitting of its ability to predict in-hospital mortality.