A Retrospective Analysis of the Performance of Intensive Care Unit Severity Scoring Systems across Different Ethnicities

Background: Despite wide utilisation of severity scoring systems for case-mix determination and benchmarking in the intensive care unit, the possibility of scoring bias across ethnicities has not been examined. Recent guidelines on the use of illness severity scores to inform triage decisions for allocation of scarce resources such as mechanical ventilation during the current COVID-19 pandemic warrant examination for possible bias in these models. We investigated the performance of three severity scoring systems (APACHE IVa, OASIS, SOFA) across ethnic groups in two large ICU databases in order to identify possible ethnicity-based bias. Method: Data from the eICU Collaborative Research Database and the Medical Information Mart for Intensive Care were analysed for score performance in Asians, African Americans, Hispanics and Whites after appropriate exclusions. Discrimination and calibration were determined for all three scoring systems in all four groups. Findings: While measurements of discrimination - area under the receiver operating characteristic curve (AUROC) - were significantly different among the groups, they did not display any discernible systematic patterns of bias. In contrast, measurements of calibration - standardised mortality ratio (SMR) - indicated persistent, and in some cases significant, patterns of difference between Hispanics and African Americans versus Asians and Whites. The differences between African Americans and Whites were consistently statistically significant. While calibrations were imperfect for all groups, the scores consistently demonstrated a pattern of over-predicting mortality for African Americans and Hispanics. Interpretation: The systematic differences in calibration across ethnic groups suggest that illness severity scores reflect bias in their predictions of mortality.