Introduction: Increasing emergency department (ED) demand and crowding has heightened focus on the need for better understanding of patient flow.

Aim: This study aimed to identify input, throughput and output factors contributing to ED patient flow bottlenecks and extended ED length of stay (EDLOS).

Method: Concurrent nested mixed method study based on retrospective analysis of attendance data, patient flow observational data and a focus group in an Australian regional ED.

Results: Analysis of 89 013 ED presentations identified increased EDLOS, particularly for patients requiring admission. Mapping of 382 patient journeys identified delays in time to triage assessment (0-39 mins) and extended waiting room stays (0-348 mins). High proportions of patients received care outside ED cubicles. Four qualitative themes emerged: coping under pressure, compromising care and safety, makeshift spaces, and makeshift roles.

Conclusion: Three key findings emerged: i) hidden waits such as extended triage-queuing occur during the input phase; ii) makeshift spaces are frequently used to assess and treat patients during times of crowding; and iii) access block has an adverse effect on output flow. Data suggests arrival numbers may not be a key predictor of EDLOS. This research contributes to our understanding of ED crowding and patient flow, informing service delivery and planning.