Purpose Computed tomography (CT) is a fast and ubiquitous tool to evaluate intra-abdominal organs and diagnose appendicitis. However, traditional CT reporting does not necessarily capture the degree of uncertainty and indeterminate findings are still common. The purpose of this study was to evaluate the reproducibility of a standardized CT reporting system for appendicitis across a large population and the system’s impact on radiologists’ certainty in diagnosing appendicitis. Methods Using a previously described standardized reporting system, eight radiologists retrospectively evaluated CT scans in a cohort of 561 patients imaged for possible appendicitis (2010–2014). Based on the original reports, 159 of these scans were selected to be over-read by a second reviewer. Reproducibility of reporting and certainty in the diagnosis of appendicitis was compared between the radiologists, and correlation made with clinical diagnoses. Results Appendicitis occurred in 20% of patients (mean age 38, 67% male) in the overall cohort (n=113 of 561). Using the standardized report, radiologists were highly accurate at identifying appendicitis (AUC=0.968, 95%CI: 0.95, 0.99), which occurred in 48% of selected exams (113 of 237). Inter-reader agreement was >80% for most objective findings, and certainty in diagnosing appendicitis was high and reproducible (AUC=0.955 and AUC=0.936 for primary readers and over-readers, respectively). In the subgroup of patients with previously “indeterminate” reports, 57% (24 of 42) were assigned the correct diagnosis using the standardized report. Conclusions Using a standardized reporting system resulted in high reproducibility of objective CT findings for appendicitis and achieved high diagnostic accuracy in an at-risk population. Predictive tools based on this reporting system may further improve communication about certainty in diagnosis and guide patient management, especially when CT findings are indeterminate.

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