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The TUMMY-UC is a Patient Reported Outcome (PRO) measure for pediatric ulcerative colitis (UC) with an observer RO (obsRO) version for children8 years. It includes eight items selected by concept elicitation interviews. We aimed to finalize the TUMMY-UC by cognitive interviews (stage-2) and to evaluate the index for its psychometric properties (stage-3).The TUMMY-UC items were first finalized during 129 cognitive debriefing interviews. Then, in a prospective-multicenter validation study, 84 children who either underwent colonoscopy or provided stool for calprotectin completed the TUMMY-UC and various measures of disease activity. Assessments were repeated after 7 and 21 days for evaluating reliability and responsiveness.During stage-2, the items were formatted with identical structure to ensure conceptual equivalence, and weighted based on ranking of importance. In stage-3, the TUMMY-UC total score had excellent reliability in repeated assessments (intraclass correlation coefficient [ICC], 0.90 [95%CI 0.84-0.94]). It also had moderate to strong correlations with all constructs of disease activity: r=0.70 with UC endoscopic index of severity, r=0.63 with IMPACT questionnaire, r=0.43 with calprotectin, r=0.80 with the PUCAI, r=0.75 with global assessment of disease activity and r=0.46 with C-reactive protein (all p0.015). The index had excellent discrimination of disease activity with a score9 defining remission (AUROC=0.95 (95%CI 0.93-0.99)). The ΔTUMMY-UC showed high responsiveness and differentiated well between children who experienced changed from those with no change.The TUMMY-UC, constructed from PRO and ObsRO versions, is a reliable, valid and responsive index, which can be now used in practice and clinical trials.