Identification of children with a nondiagnostic ultrasound at a low appendicitis risk using a pediatric Appendicitis Risk Calculator

Objectives: Up to 50% of ultrasounds (USs) for suspected pediatric appendicitis are nondiagnostic. While the validated low-risk clinical pediatric Appendicitis Risk Calculator (pARC) score < 15% and the low-risk US with nonvisualized appendix and no periappendiceal inflammation carry relatively low appendicitis risks, the contribution of the combination of both characteristics to this risk has never been assessed. The primary objective was to determine the proportion of children with the low-risk US-low-risk pARC combination with appendicitis. We hypothesized that this proportion would be 2.5% (upper 95% CI <= 5%). Methods: A retrospective cohort study of 448 previously healthy children 4-17 years old at a pediatric ED with suspected appendicitis, nondiagnostic US, and persistent clinical concern about appendicitis. Two investigators abstracted demographic, clinical, and imaging data. Based on published criteria, USs were classified as low-risk or high-risk. The pARC includes seven demographic, clinical, and laboratory variables and is quantified according to the published formula. The primary outcome was appendicitis, based on the histological evidence. All nonoperated patients underwent a 1-month-follow-up to exclude delayed appendicitis diagnoses. Results: Sixty of the 448 (13.4%) patients had appendicitis; 269 (60%) had low-risk US, 262 (58.4%) had low-risk pARC, and 163 (36.4%) had both characteristics. The appendicitis rates with low-risk pARC alone and low-risk US alone were 14/262 (5.4%) and 21/269 (7.8%), respectively. A total of 2/163 children (1.2%) with low-risk pARC and low-risk US had appendicitis (95% CI 0%-4.4%). Higher-risk US increased the appendicitis odds 5 (95% CI 1.54-20.55) to 11 times (95% CI 2.41-51.10) across pARC levels. The low-risk combination had sensitivity of 96.7% (95% CI 88.5%-99.6%), specificity of 41.5%, positive predictive value of 20.4%, and negative predictive value of 98.8% (95% CI 95.6%-99.9%). Conclusions: The children with low-risk pARC and low-risk US combination are unlikely to have appendicitis and can be discharged home. The presence of higher-risk US-pARC score combinations substantially increases the appendicitis risk and warrants reassessment or interval imaging.