Shari J. Welch, MD; James Augustine, MD; Li Dong, MD, MStat; Lucy Savitz, MBA, PhD; Gregory Snow, PhD; Brent C. James, MD E mergency departments (EDs) are an important source of care for a large segment of the population of the United States. There are more than 131 thirty million visits to the ED each year, and two-thirds of hospital admissions are via the ED. 1,2 In spite of its importance, the ED is widely considered by pa- tients and providers as a place of inefficient, unsatisfactory, and often unsafe health care encounters. 3,4 Almost 40% of patients wait more than an hour to see a physician, and a quarter of the patients seen in the ED spend more than four hours there. 1 Waits and delays lead to inefficient patient flow and crowding, factors linked to adverse clinical outcomes. 5–11 This ED milieu also neg- atively affects access to care and is associated with 2% of patients (2.6 million persons annually) leaving the ED without being seen by a physician. 1 Measurement and monitoring of emergency department per- formance has been prompted by The Joint Commission’s patient flow standards, 12,13 and the Centers for Medicare and Medicaid Services (CMS) performance metrics. 14,15 Data involving length of stay (LOS), left without being seen (LWBS), and door-to- physician times are currently being reported to CMS and will be factored into Value-Based Purchasing payment reform for the first time in 2013 and 2014. 16 However, in contrast to typical evidence-based clinical bench- marks, comparison strategies for ED operational performance have been harder to establish because of substantial differences between EDs in terms of volume and acuity. EDs exhibit and differ across an array of operating character- istics including those which reflect acuity (such as admission rate, high Current Procedural Terminology [CPT] acuity rate, and pediatric patient percentage). Leaders and managers of EDs are concerned that comparisons made between EDs do not take into account such differences. To address this issue, stakeholders from the emergency medicine and quality improvement communities have suggested comparison schemes developed by expert con- sensus, although data to support such schemes have been lack- ing. 17–21 Performance capability of large groups of EDs have been Performance Measures Volume-Related Differences in Emergency Department Performance Article-at-a-Glance Background: Emergency departments (EDs) are an im- portant source of care for a large segment of the population of the United States. There are more than 131 thirty million visits to the ED each year, and two-thirds of hospital admis- sions are via the ED. Measurement and monitoring of emer- gency department performance has been prompted by The Joint Commission’s patient flow standards. A study was con- ducted to attempt to correlate ED volume and other oper- ating characteristics with performance on metrics. Methods: A retrospective analysis of the Emergency Depart- ment Benchmarking Alliance annual ED survey data for the most recent year for which data were available (2009) to ex- plore observed patterns in ED performance relative to size and operating characteristics. The survey was based on 14.6 mil- lion ED visits in 358 hospitals of varying sizes across the United States, with an ED size representation (sampling) approximat- ing that of the Emergency Medicine Network (EM Net). Results: Larger EDs (with higher annual volumes) had longer LOSs (p < .0001) and higher LBTC rates (p < .0001), indicating poorer operational performance. Larger EDs had longer door-to-physician times as well (p < .0012). Operat- ing characteristics indicative of higher acuity were associated with worsened performance on metrics and lower acuity characteristics with improved performance.. Conclusion: ED volume, which also correlates with many operating characteristics, is the strongest predictor of oper- ational performance on metrics and can be used to catego- rize EDs for comparative analysis. Operating characteristics indicative of acuity also influence performance. The find- ings suggest that ED performance measures should take ED volume, acuity, and other characteristics into account and that these features have important implications for ED de- sign, operations, and policy decisions. The Joint Commission Journal on Quality and Patient Safety September 2012 Volume 38 Number 9 395