Development and calibration of a simple mortality risk score for hospitalized COVID-19 adults

Objectives: Mortality risk scores, such as SOFA, qSOFA, and CURB-65, are quick, effective tools for communicating a patient9s prognosis and guiding therapeutic decisions. Most use simple calculations that can be performed by hand. While several COVID-19 specific risk scores exist, they lack the ease of use of these simpler scores. The objectives of this study were (1) to design, validate, and calibrate a simple, easy-to-use mortality risk score for COVID-19 patients and (2) to recalibrate SOFA, qSOFA, and CURB-65 in a hospitalized COVID-19 population. Design: Retrospective cohort study incorporating demographic, clinical, laboratory, and admissions data from electronic health records. Setting: Multi-hospital health system in New York City. Five hospitals were included: one quaternary care facility, one tertiary care facility, and three community hospitals. Participants: Patients (n=4840) with laboratory-confirmed SARS-CoV2 infection who were admitted between March 1 and April 28, 2020. Main outcome measures: Gray9s K-sample test for the cumulative incidence of a competing risk was used to assess and rank 48 different variables9 associations with mortality. Candidate variables were added to the composite score using DeLong9s test to evaluate their effect on predictive performance (AUC) of in-hospital mortality. Final AUCs for the new score, SOFA, qSOFA, and CURB-65 were assessed on an independent test set. Results: Of 48 variables investigated, 36 (75%) displayed significant (p<0.05 by Gray9s test) associations with mortality. The variables selected for the final score were (1) oxygen support level, (2) troponin, (3) blood urea nitrogen, (4) lymphocyte percentage, (5) Glasgow Coma Score, and (6) age. The new score, COBALT, outperforms SOFA, qSOFA, and CURB-65 at predicting mortality in this COVID-19 population: AUCs for initial, maximum, and mean COBALT scores were 0.81, 0.91, and 0.92, compared to 0.77, 0.87, and 0.87 for SOFA. We provide COVID-19 specific mortality estimates at all score levels for COBALT, SOFA, qSOFA, and CURB-65. Conclusions: The COBALT score provides a simple way to estimate mortality risk in hospitalized COVID-19 patients with superior performance to SOFA and other scores currently in widespread use. Evaluation of SOFA, qSOFA, and CURB-65 in this population highlights the importance of recalibrating mortality risk scores when they are used under novel conditions, such as the COVID-19 pandemic. This study9s approach to score design could also be applied in other contexts to create simple, practical and high-performing mortality risk scores.