Prospective Multicenter External Validation of the BIDIAP Index for the Diagnosis of Pediatric Acute Appendicitis

Introduction: Pediatric acute appendicitis (PAA) remains challenging to diagnose despite existing diagnostic scores. The BIDIAP index is a three-item diagnostic tool with very high discriminative performance in a derivation cohort. This study aimed to prospectively and externally validate the BIDIAP index in a multicenter pediatric population. Material and Methods: We conducted a prospective, multicenter observational study across four tertiary pediatric centers, enrolling children presenting with suspected PAA. Two groups were analyzed: patients with histopathologically confirmed PAA and patients in whom appendicitis was confidently excluded after diagnostic work-up, classified as non-surgical abdominal pain (NSAP). The BIDIAP index was applied using a predefined cutoff (≥ 4 points), and diagnostic performance was assessed using ROC analysis, calibration metrics, and decision curve analysis (DCA). Results: A total of 644 patients meeting the prespecified analytical criteria were included in the primary analysis. The BIDIAP index demonstrated excellent diagnostic performance, with an area under the ROC curve of 0.93 (95% CI, 0.92–0.95). The calibration slope was 1.00, and the intercept was close to zero, indicating close agreement between predicted and observed risks. At the prespecified cutoff value of ≥ 4 points, the BIDIAP index achieved a sensitivity of 89.89% and a specificity of 83.21%. DCA showed a positive net clinical benefit of the BIDIAP index over treat-all and treat-none strategies across the full range of clinically relevant threshold probabilities. Conclusions: The BIDIAP index demonstrated excellent diagnostic performance for PAA. Its simplicity, based on only three items, and its potential applicability even when the appendix is not visualized on ultrasonography make the BIDIAP index a promising tool for supporting clinical decision-making in routine pediatric emergency practice.