Objective Patient safety studies have identified older adults as a high-risk group for adverse events (AEs). As frequent users of the emergency department (ED), they are vulnerable to the negative consequences of ED crowding. The study objective was to determine whether a prolonged ED stay is associated with an increased risk for the occurrence of AEs for older patients admitted to hospital.

Methods This retrospective cohort study was conducted at the largest adult tertiary care facility in Atlantic Canada (1 July 2005–31 March 2006). All community-dwelling persons 65 years and older admitted to an acute inpatient unit from the ED were eligible. The exposure of interest was total length of stay (LOS) in the ED. The primary outcome was the occurrence of an AE in-hospital. AEs were identified from administrative diagnostic data using previously validated screening criteria.

Results The average age of 982 eligible participants was 77.8 years (SD 7.8). The majority (75.0%) experienced a prolonged ED LOS as defined by national guidelines. There was evidence of at least one AE in 140 (14.3%) records. After adjustment, for every hour spent in the ED, the odds of experiencing an AE in-hospital increased 3% (OR 1.03, 95% CI 1.004 to 1.05). Those with an AE had twice the hospital LOS (20.2 vs 9.8 days, p<0.00001).

Interpretation A prolonged ED stay for older admitted patients is associated with an increased risk of an in-hospital AE. The longer hospital LOS associated with AEs further reduces the availability of acute care beds, thus exacerbating ED crowding.