Background: Emergency department (ED) crowding and prolonged waiting times have been associated with adverse consequences towards quality and patient safety. Objective: This study investigates whether the number of patients simultaneously present at the ED might be an indicator of unsafe waiting and at what threshold hospital-wide measures to improve patient outflow could be justified. Methods: Data were retrospectively collected during a 1-year period; all ED patients aged P16 years, and

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... d as ESI-1 or ESI-2 were eligible for inclusion. The number of patients simultaneously present was used as occupancy rate. Waiting time was considered unsafe if it was longer than 10 min for ESI-1 patients, or longer than 30 min for ESI-2 patients. Differences in waiting time and occupancy between patients with safe and unsafe waiting times were analysed using the Mann-Whitney U test. The ability of the occupancy rate to discriminate unsafe waiting times was analysed using a receiver operating characteristic curve. Results: The overall median waiting time was 5 min (IQR = 4-8) for ESI-1, and 12 min (IQR = 6-24) for ESI-2 patients. Unsafe waiting times occurred in 16.0% of ESI-1 patients (median waiting time = 17 min, IQR = 13-23), and in 18.9% of ESI-2 patients (median waiting time = 48 min, IQR = 37-68). The occupancy rate was a weak indicator for unsafe waiting times in ESI-1 patients (AUC = 0.625, 95%CI 0.537-0.713) but a fair indicator for unsafe waiting times in ESI-2 patients (AUC = 0.740, 95%CI 0.727-0.753) for which the threshold to predict unsafe waiting times with 90% sensitivity was 51 patients. Conclusion: The number of patients simultaneously present is a moderate indicator of unsafe waiting times. Future initiatives to improve safe waiting times should not focus solely on occupancy, and expand their focus towards other factors affecting waiting time.