Background & aimsNo fully validated indices to measure pouchitis activity exist. We aimed to develop and externally validate a novel endoscopic and histologic index.MethodsEndoscopists and pathologists used 11 (4 endoscopic, 4 histologic, 3 composite) existing indices and items from a prior Research and Development/University of California Los Angeles appropriateness exercise to assess pouchitis disease activity in videos and images from 98 patients with chronic antibiotic-refractory pouchitis who participated in a randomized placebo-controlled alicaforsen trial. Reliability was assessed with the intraclass correlation coefficient (ICC). Responsiveness was quantified by the area under the receiver operating characteristic curve (AUROC). A novel index was developed using linear regression with a visual analog scale (VAS) of pouchitis endoscopic and histologic disease activity as the dependent variable and was externally validated with the EARNEST vedolizumab trial data.ResultsThe Atlantic Pouchitis Index (API) (range, 0-69) comprises the Simple Endoscopic Score for Crohn's Disease and Robarts Histopathology Index. The API exhibited almost perfect intra-rater (ICC, 0.88; 95% confidence interval [CI], 0.81-0.92) and substantial inter-rater reliability (ICC, 0.72; 95% CI, 0.60-0.79). A high degree of responsiveness (AUROC, 0.95; 95% CI, 0.89-0.98), greater than existing endoscopic indices (ΔAUROC, 0.09-0.24; P ≤ .005), was observed when the change criterion was a decrease in the VAS of one-half of the standard deviation. Good responsiveness was observed in external validation when vedolizumab was the change criterion (AUROC, 0.63; 95% CI, 0.51-0.73).ConclusionsDevelopment and validation of the API advances pouchitis disease assessment. Integration of endoscopy and histopathology results in an objective, reliable, and responsive instrument to evaluate therapy effectiveness in research and clinical settings.