Background and ObjectiveFor people with cystic fibrosis, validated patient-reported outcome measures for the assessment of the complex abdominal involvement are lacking. The objective of this study was to examine whether the CFAbd-Score, a novel questionnaire consisting of 28 items, meets the essential requirements (validity and reliability) for a patient-reported outcome measure according to US Food and Drug Administration recommendations. Methods Content validity was assessed by recording the frequencies and severity of symptoms that occurred during the prior 2 weeks in patients with cystic fibrosis (n = 116; aged ≥ 6 years). Comparing the CFAbd-Score results obtained from patients with cystic fibrosis and healthy controls (n = 88), we determined known-groups validity. To explore the structure of the patient-reported outcome measure, a factor analysis was conducted. Internal consistency of the five extracted score domains was assessed using Cronbach's alpha. For test-retest reliability, a subgroup of patients (n = 43) was reevaluated and intra-class correlation coefficients were determined. ResultsThe CFAbd-Score differentiated patients with cystic fibrosis from healthy controls with a large effect size (17.3 ± 1.1 vs. 8.0 ± 0.7 points; p < 0.001; Cohen's d = 0.85). Items, domains, and scores reflected the relevance to patients with cystic fibrosis and allowed a differentiation between subgroups of patients with cystic fibrosis (e.g., patients with and without abdominal pain, pancreatic sufficiency, and age groups). High item-domain loadings as well as good to excellent internal consistency and reproducibility (Cronbach's α = 0.70-0.92; intra-class correlation coefficient = 0.932, 95% confidence interval 0.874-0.963) indicated construct validity and reliability. Conclusions The CFAbd-Score has successfully passed through key steps of the iterative process of patient-reported outcome measure development. Prospectively, the CFAbd-Score is proposed as a patient-centered instrument for monitoring abdominal symptoms and, most interestingly, for evaluating changes in symptoms with novel treatments such as cystic fibrosis transmembrane regulator modulators. Trail Registration ClinicalTrials.gov: NCT03052283.

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