Association Between In-Hospital Mortality and ICU Patient Volume at the Facility-Level Stratified by Severity

Objectives: Large-volume hospitals for critically ill patients are generally associated with improved outcomes. However, the effect of ICU volume together with patient severity remains insufficiently evaluated. Therefore, the present study aimed to determine the impact of ICU patient volume stratified by Sequential Organ Failure Assessment (SOFA) scores (0–24) on mortality and clarify the possible effect of volume within a specific severity range. Design: Retrospective observational study using a nationwide Japanese database. Setting: Japanese Diagnosis Procedure Combination inpatient database. Patients: A total of 568,340 adult patients were admitted to the ICU in 430 hospitals between October 1, 2020, and March 31, 2023. Interventions: None. Measurements and Main Results: The primary outcome was in-hospital mortality. We calculated the facility-level annual patient volume above a SOFA score threshold of X (range: 0–24) and examined its association with mortality using multilevel mixed-effects logistic regression models. Although total ICU volume was not significantly associated with mortality (adjusted odds ratio [OR] per 10-patient increase: 0.999; 95% CI, 0.999–1.000; p = 0.147), the volume of ICU patients above a SOFA score threshold of 5 was significantly associated with decreased in-hospital mortality (OR, 0.997; 95% CI, 0.995–0.998; p < 0.001). Overall, the adjusted ORs progressively decreased as the SOFA score threshold increased, indicating an enhanced inverse association at increased severity levels. Conclusions: Increased volumes of patients with severe illnesses were associated with decreased in-hospital mortality rates; this inverse volume-outcome relationship became more pronounced as the severity threshold increased.