Abstract. 
Background Medical imaging use has increased progressively, prompting discussions about its clinical impact. Interventions to reduce low-value imaging have had varying success, as they generally do not consider the influence of the clinical environment on decision-making. Factors affecting imaging ordering decisions by Emergency Department (ED) medical officers (MOs) and how these factors differ between day and night shifts are poorly understood.
Methodology This mixed methods study was conducted in 2021 at a major tertiary hospital in Western Sydney. Observations and interviews with ED MOs for 20 day-shift and 26 night-shift clinical encounters were analysed to understand how and why imaging decisions were made, along with usage of imaging guidelines. Demographic and clinical patient data (including patient disposition) were obtained retrospectively from medical records to assess the impact of imaging.
Results During night shifts, 18 of the 26 observed clinical encounters used diagnostic imaging, compared with 12 of the 20 observed clinical encounters during day shifts. Factors affecting decision-making during night shifts included limited resources, fatigue, reduced support for junior ED MOs and higher patient load. Interviews suggested CT was more likely to be used during night shifts as a screening tool to expedite decisions and as a substitute for unavailable imaging modalities. In contrast, imaging decisions by day shift junior MOs were influenced by the need to justify their decisions to senior MOs, prompting them to research presenting complaints and imaging indications. Generally, there was minimal reference to imaging decision-making guidelines across both shifts.
Conclusion Differing factors impact imaging decisions by ED MOs during day and night shifts. This needs consideration when designing and implementing targeted physician support strategies and interventions to reduce low-value imaging. Limited resources and MO fatigue should be considered when modifying guidelines/strategies aiming to support MOs during ED night shifts.
