ACADEMIC EMERGENCY MEDICINE 2011; 18:941–946 © 2011 by the Society for Academic Emergency Medicine


Objectives:  The primary study aim was to examine the variations in crowding when an emergency department (ED) initiates ambulance diversion.

Methods:  This retrospective, multicenter study included nine geographically disparate EDs. Daily ED operational variables were collected during a 12-month period (January 2009 to December 2009), including total number of ED visits, mean overall length of stay (LOS), number of ED beds, and hours on ambulance diversion. The primary outcome variable was the “ED workload rate,” a surrogate marker for daily ED crowding. It was calculated as the total number of daily ED visits multiplied by the overall mean LOS (in hours) and divided by the number of ED beds available for acute treatment in a given day. The primary predictor variables were ambulance diversion, as a dichotomous variable of whether or not an ED went on diversion at least once during a 24-hour period, diversion hour quintiles, and sites.

Results:  The annual ED census ranged from 43,000 to 101,000 patients. The percentage of days that an ED went on diversion at least once varied from 4.9% to 86.6%. On days with ambulance diversion, the mean ED workload rate varied from 17.1 to 62.1 patient LOS hours per ED bed among sites. The magnitude of variation in ED workload rate was similar on days without ambulance diversion. Differences in ED workload rate varied among sites, ranging from 1.0 to 6.0 patient LOS hours per ED bed. ED workload rate was higher on average on diversion days compared to nondiversion days. The mean difference between diversion and nondiversion was statistically significant for the majority of sites.

Conclusions:  There was marked variation in ED workload rates and whether or not ambulance diversion occurred during a 24-hour period. This variability in initiating ambulance diversion suggests different or inconsistently applied decision-making criteria for initiating diversion.