Emergency department crowding associated with increased 30‐day mortality: a cohort study in Stockholm Region, Sweden, 2012 to 2016

Objectives Emergency department (ED) crowding is a major problem across the world. Studies investigating the association between crowding and mortality are many, but the quality is inconsistent and there are very few large, high‐quality multicenter studies that are properly designed to deal with confounding due to case mix. The aim of this study is to investigate the association between ED crowding and 30‐day mortality. Methods We conducted an observational cohort study at all 7 EDs in Stockholm Region, Sweden 2012–2016. The crowding exposure was defined as the mean hourly ED census during the shift that the exposed patient arrived, divided with the expected ED census for this shift. The expected ED census was estimated using a separate linear model for each hospital with year and shift as predictors. The exposure was categorized in 3 groups: reference (lowest 75% of observations), moderate (75%–95% of observations), and high (highest 5% of observations). Hazard ratios (HR) for all‐cause mortality within 30 days were estimated with a Cox proportional hazards model. The model was adjusted for age, sex, triage priority, arrival hour, weekend, arrival mode, chief complaint, number of prior hospital admissions, and comorbidities. Results 884,228 patients who visited the ED 2,252,656 times were included in the analysis. The estimated HR (95% confidence interval) for death within 30‐days was 1.00 (0.97–1.03) in crowding category 75%‐95% and 1.08 (1.03–1.14) in the 95%‐100% category. Conclusions In a large cohort study including 7 EDs in Stockholm Region, Sweden we identified a significant association between high levels of ED crowding and increased 30‐day mortality.