221 Low-dose-CT versus X-ray in patients suspected of pulmonary disease with possible pulmonary congestion and pneumothorax for CXR. The higher number of patients with a baseline comorbidity of asthma in the ULDCT group, and the resulting higher number of patients with a day 28 diagnosis of asthma exacerbation in the ULDCT group, are probably due to a seasonal increase of asthma exacerbations in the months February, March, and April, when ULDCT was more often the allocated method [27]. However, the similarity in baseline characteristics and presenting symptoms in both study groups indicates that this was unlikely to result in a systematic bias in our study. The SF-12 questionnaire response (ULDCT 77.2%; CXR 71.6%) was, despite many efforts, lower than anticipated but higher than in earlier studies at the ED [28, 29]. However, there were only minor differences in baseline characteristics between responders and non-responders (Supplementary Tables 2 and 3). We designed this study as a non-inferiority trial, since we anticipated that the superior accuracy of ULDCT would lead to health outcomes at least as good as after CXR, with a more efficient health care process. The point estimate of the mean difference between the two groups shows a 1.1 point difference in mean PCS score in favour of ULDCT. With more returned questionnaires, the precision in this estimate would have been larger and power increased, potentially demonstrating statistically significant superiority of the ULDCT strategy. However, we believe that as difference of 1.1 in mean the PCS-score found is close to the pre-specified non-inferiority margin of 1 point and unlikely to be of clinical relevance. Multiple radiologists with different levels of experience in chest imaging were involved in reading the ULDCT and CXR examinations for this study. This has caused inter-reader variability, although the low proportion of initial reports adjusted by the supervisor (1.0% ULDCTs, 0.6% CXRs) shows that reporting was largely consistent. This study was performed before the outbreak of the corona virus infectious disease-19 (COVID-19) pandemic. The pandemic changed the incidence, presentation and management of patients suspected of CAP compared to the usual situation, which is not accounted for in our study [30]. To our knowledge, this large-scale pragmatic randomised trial is unique in its aim to assess the yield of replacing CXR by ULDCT in the diagnostic work-up of ED patients suspected of non-traumatic pulmonary disease, not in terms of diagnostic accuracy, but in terms of short-term patient outcomes and health care efficiency. Participants were included in an urban university hospital and a suburban teaching hospital ensuring an unselected, mixed study population, making results widely applicable. The development and use of the diagnostic handbook adds to the consistency of disease classification and can also be considered one of the strengths of our approach [20]. Improved diagnostic accuracy on itself does not automatically translate to improved patient outcomes, as the impact of imaging depends on the outcome of clinical interventions that follow [16]. It is therefore argued that significant changes in treatment planning and a meaningful change in patient outcome should be documented for new radiologic applications to be accepted [31]. Our study showed that ULDCT leads to no marked effect on health care efficiency, in terms of number of admissions and hospital length of stay or on short-term health outcomes, in the broad population of ED patients suspected of non-traumatic pulmonary disease. Furthermore, ULDCT has higher immediate imaging costs than CXR while both the examination and reading require more time, which might be a disadvantage at a busy ED. The findings in this 9
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