Does Replacing SOFA-1 with SOFA-2 Change Mortality Discrimination and Calibration Across Patient Subgroups? A Paired Analysis in MIMIC-IV

Background. The Sequential Organ Failure Assessment (SOFA) score describes organ dysfunction in intensive care. SOFA-2, published in 2025, updates the 1996 score (SOFA-1). It is unknown whether the update changes mortality discrimination and calibration within patient subgroups. Methods. We analyzed 85,228 first ICU stays per hospitalization (65,354 adults) in MIMIC-IV v3.1. We scored SOFA-1 (two prespecified conventions for intubated patients) and SOFA-2 from the same data over the same first 24 hours. The primary outcome was in-hospital mortality; ICU mortality was secondary. Using a paired patient-level bootstrap (5,000 replicates), we estimated AUROC overall and by sex, race and insurance; the paired change between scores; subgroup gaps in AUROC and the change in those gaps; and descriptive calibration. The protocol was committed before any analysis relating scores to mortality. Results. In-hospital death occurred in 9,468 stays (11.1%). AUROC was 0.762 for SOFA-2, 0.743 for SOFA-1 as charted and 0.735 for SOFA-1 under the community convention; SOFA-2 minus SOFA-1 was +0.019 (+0.016 to +0.023) and +0.027 (+0.023 to +0.031). Swapping single domains showed that the respiratory domain accounted for the whole gain, about 74% of it through the SpO2/FiO2 fallback. Subgroup gaps changed against SOFA-1 as charted (male-female +0.021; White-Black +0.025), driven by the brain-domain rule, but not against the community convention. The White-Black change was not seen in patients under 65, and neither change was consistent across ICU types. Calibration differences between groups were large but changed little between scores (|Δ log O/E| ≤ 0.045). Insurance-group differences largely disappeared in patients under 65, whereas race-group differences did not. Conclusions. SOFA-2 modestly improved overall discrimination for in-hospital mortality, mainly through its respiratory domain. Changes in subgroup performance gaps were small. They depended on how SOFA-1 handles intubated patients, partly reflected age, and were not consistent across ICU types. These are descriptive associations in a single database; we draw no causal or fairness conclusions.