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