ED crowding has been reported to reduce the quality of care. There are many proposed crowding metrics, but the metric most strongly associated with care quality remains unknown. The present study aims to determine the crowding metric with the strongest links with processes and outcomes of care linked to the Institute of Medicine quality domains. Systematic searches in healthcare databases were conducted using terms for ‘crowding’, ‘metrics’ and ‘performance’, supplemented by grey literature and citation searches. The level of evidence for each association was assessed using an explicit tool. The body of evidence was assessed using the Grading of Recommendations Assessment, Development and Evaluation approach. Evidence was synthesised using harvest plots. Titles and abstracts of 2052 studies were screened, 452 selected for full-text review and 183 included. Inter-observer agreement was moderate κ = 0.54 (95% confidence interval 0.50–0.59). Two thirds were from urban tertiary hospitals in North America (65%), Australasia (13%), Europe (12%) and Asia (8%). One third provided Level 3 or higher evidence. Metrics were based on occupancy (38%), time (31%), workload (19%) or combinations (9%). Data were synthesised from 25 607 375 patients, 2368 staff, 9089 hospitals and 101 177 sampling times. Almost all crowding metrics were patient-centred and reflect timeliness and efficiency. ED length of stay, boarding time and total occupancy had the strongest association with safety and effectiveness of care. ED length of stay was also associated with equity. The certainty of evidence for associations between crowding measures varied across domains of quality, from very low to moderate certainty.
Crowding measures with the strongest associations with quality of care were total ED occupancy and total EDLOS (measured as a departmental average rather than at individual level, and in higher acuity settings).
Occupancy varies from hour to hour and is a useful measure to trigger real-time interventions to mitigate immediate harm to patients. Whereas EDLOS at departmental level may be a more reliable measure of overall system performance over time and be more suited as a quality indicator at state or national level to drive long term system changes.