Postpandemic Recovery of Case Mix Index and Risk-Adjusted Mortality in US Hospitals

Importance: The COVID-19 pandemic disrupted long-standing trends in hospital quality and patient safety, prompting questions about whether risk-adjusted outcomes have resumed their prepandemic trajectories. Objective: To determine whether trends in risk-adjusted mortality and case mix index (CMI) among hospitalized patients in hospitals have returned to prepandemic trajectories. Design, Setting, and Participants: This retrospective cohort study was a multicenter analysis that used the Vizient Clinical Data Base, which contains encounter-level administrative and financial data from more than 1300 hospitals in the US. The study included continuously reporting hospitals and examined inpatient discharges between October 2019 and March 2024. Data were analyzed between January and May 2025. Main Outcomes and Measures: Quarterly estimates of CMI and standardized mortality ratio (SMR) (observed-to-expected ratio). Ordinary least squares (OLS) regression was used to evaluate overall linear trends, and joinpoint regression was used to identify inflection points. Statistical significance was defined as P < .05, with slope estimates reported alongside 95% CIs. Results: Among 715 hospitals and 7 802 606 million inpatient encounters, the mean CMI increased from 1.70 in the fourth quarter (Q4) of 2019 to 1.79 in the first quarter (Q1) of 2024 (difference, 0.09; 95% CI, 0.01 to 0.17; P = .02). In OLS regression, CMI showed no significant overall linear trend (R2 = 0.006; P = .77). The mean SMR decreased from 1.00 in Q4-2019 to 0.80 in Q1-2024 (difference, -0.20; 95% CI, -0.32 to -0.08; P = .001), with a significant linear decline across the study period (R2 = 0.735; P < .001). Joinpoint regression identified a CMI inflection point in Q4-2020 (slope, 1.85; 95% CI, 0.73 to 4.14; P < .001 before; slope, -0.30; 95% CI, -0.62 to -0.09; P = .006 after) and an SMR inflection point in Q3-2021, after which SMR declined significantly by -3.17% per quarter (P < .05). Conclusions and Relevance: In this cohort study of 715 US hospitals from 2019 to 2024, risk-adjusted in-hospital mortality declined significantly following the COVID-19 pandemic, resuming its prepandemic trajectory of improvement, while patient acuity as measured by CMI remained elevated. These findings suggest a new postpandemic baseline for patient acuity, whereas hospital mortality outcomes have returned to prior improvement trends.