A Cross-Sectional Study on the Impact of Emergency Department Boarding and Crowding on Emergency Department Observation Unit Patient Populations.

Objectives: Emergency Department (ED) boarding affects patient flow and clinical decision making, but its impact on ED observation unit (EDOU) utilization remains unclear. In this study, we examine whether ED boarding census levels are associated with changes in the characteristics, care processes, and outcomes of patients placed in the EDOU. Methods: We performed a cross-sectional study of all adult EDOU encounters from July 1, 2023 to June 30, 2024 in a single academic ED (∼120,000 annual visits) with a 31-bed EDOU staffed by ED clinicians. Encounters were stratified into quartiles of ED boarding census (number of admitted patients boarding ≥2 hours). Patient demographics, encounter characteristics, resource utilization, and outcomes were compared using χ Results: Among 8,469 encounters, patient characteristics remained largely consistent across boarding quartiles. Most diagnostic testing and imaging did not vary significantly. Higher boarding levels were associated with longer ED-to-EDOU times and modest increases in specialty consultations, ultrasounds, and echocardiograms. Admission rates were slightly higher in the highest boarding quartile compared with the lowest. EDOU length of stay increased incrementally with higher boarding levels. Conclusions: ED boarding levels were not strongly associated with changes in EDOU patient characteristics, management, or outcomes. Structured eligibility criteria and dedicated review processes likely insulated EDOU use from short-term operational pressures. Modest increases in consultations, diagnostics, and admissions suggest that slightly more complex patients may be placed in the EDOU during periods of high boarding. Overall, EDOU function remained relatively stable, supporting its role as a consistent throughput mechanism even during ED crowding.