The probability of emergency department diversion status as a function of inpatient occupancy

Abstract

Study objectives: Emergency department (ED) diversion status is a result of a mismatch between ED capacity and various input, throughput, and output factors. It is important to measure these factors to manage ED flow and predict ambulance diversion. The most important output factor for most EDs is the inpatient medical-surgical occupancy rate, which varies considerably but predictably throughout the week. The medical-surgical occupancy rate was calculated as the sum of the number of patients occupying beds at midnight and the number of discharged patients in the previous 24-hour period divided by the total number of staffed beds. We measure the odds and probability of daily diversion status according to variability in the following daily variables: (1) medical-surgical inpatient occupancy; (2) ED volume; (3) number of ED admissions; (4) number of ED ICU admissions; (5) number of ED clinical attending physician hours and shifts; (6) ED throughput time (TPT); (7) day of week; and (8) number of elective operating room surgical cases. Methods: Data were collected retrospectively from ED and hospital electronic logs for 24-hour periods starting at midnight for 102,327 ED visits between April 15, 2002, and February 29, 2004. The association between ED diversion and each independent variable was measured in univariate analysis by Student's t tests. Variables reaching statistical significance at P value less than .05 were further analyzed in a multiple logistic regression model. Results: The baseline probability of diversion was 36%. The following 3 variables reached statistical significance in univariate analysis: (1) medical-surgical occupancy rate; (2) day of the week; and (3) TPT. In multivariate analysis, the odds ratios and increased probability of diversion status were as follows: 1.2 and 4% for each 5% increase in medical-surgical occupancy; 2.0 and 17% when TPT was greater than or equal to the median of 238 minutes; and 1.8 and 15% for weekdays compared with weekends. Conclusion: The probability of ambulance diversion on a weekday with a TPT greater than or equal to the median of 238 minutes was 68%. Increasing inpatient capacity on weekdays is likely to have a positive effect on decreasing ambulance diversion. Variability in the number of elective surgical cases was not associated with diversion status.