Original research
Figure 1 DAShED participant recruitment.
the intervals that are usually reported in pilot and feasibility studies. 22 However, for the diagnostic accuracy analysis of CDTs, we used the conventional 95% CIs. Statistical analysis A descriptive analysis was performed. Categorical variables were summarised using frequencies and percentages and continuous variables using medians and IQRs. Exact binomial 80% CIs were constructed around key proportions such as the proportion of patients in whom the clinician considered AAS was not a possible differential. CDTs were assessed for diagnostic accuracy, taking a confirmed AAS diagnosis as the reference standard. Reference standard results were not available to those providing data to derive the CDTs, and conversely, information on decision tools were not readily available to those making a confirmed AAS diagnosis. The following performance indices were calculated (all with exact binomial 95% CIs): sensitivity, specificity, positive predic- tive value and negative predictive value. Total scores were calculated for the CDTs by using the published criteria. 15–17 If more than 50% of the individual dimensions of the CDT had more than 50% missing data, then a total score was not calculated. Where scores were able to be calculated, any missing data were scored as 0. The presence of missing data in the total scores and refer- ence standard (confirmed AAS diagnosis) may have caused bias
► Median time from symptom onset to hospital presentation. ► 30-day outcome (including 30-day mortality in proven AAS) through EPR. Outcomes Each local study team kept a record on a password protected National Health Service computer linking the patient’s hospital number with their study number so that 30-day outcome data could be collected from the EPR and entered onto the eCRF noting any diagnosis of AAS, final hospital diagnosis and mortality. Reference standard was radiological or operative confirmation of AAS. Sample size calculation The sample size calculation was based on the expected width of 80% CIs. We estimated that approximately 5000 eligible patients would attend during the study period, of whom 125 would undergo CTA and 6 would have confirmed AAS. This would provide sufficient power to estimate key measures with an acceptable degree of precision, for example, 0.12% preva- lence of confirmed AAS (80% CI 0.06% to 0.21%); 2.5% prev- alence of CTA use (80%CI 2.2% to 2.8%). We used 80% CIs for the key proportions so that we were not reporting intervals that were too wide to be informative, while also enabling us to identify the most likely range of values for the true proportion with reasonable certainty. Indeed, 80% CIs are consistent with
McLatchie R, et al . Emerg Med J 2024; 41 :136–144. doi:10.1136/emermed-2023-213266
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