Objectives:​ To assess the reliability and validity of age-specific versions of the Fibrodysplasia Ossificans Progressiva Physical Function Questionnaire (FOP-PFQ), developed to measure the impact of FOP on physical function and activities of daily living.

Methods:​ FOP-PFQ development included a literature review, two iterative phases of qualitative work involving individuals with FOP, and clinical expert review. The analysis used pooled FOP-PFQ data from an FOP natural history study (NCT02322255), a patient registry (NCT02745158), and phase II trials (NCT02190747;​ NCT02279095;​ NCT02979769). Item-level and factor analysis informed item retention and determined factor structure. Reliability was evaluated using Cronbach's alpha and intraclass correlation coefficients. Convergent validity was assessed by comparing scores with age, the Cumulative Analogue Joint Involvement Scale (CAJIS), the Patient-Reported Outcomes Measurement Information System Global Health Scale (PROMIS), and heterotopic ossification (HO) volume. Known-groups validity assessment used age, CAJIS, and HO volume.

Results:​ Factor analysis confirmed a two-factor solution:​ Mobility and Upper Extremity. Results reflected high internal consistency and were supportive of test-retest reliability;​ correlation coefficients >0.90 demonstrated FOP-PFQ scores were stable over a one- to three-week period. The majority of scores were moderately (r = 0.30–0.50) to highly (r ≥ 0.50) correlated with CAJIS and HO volume, supporting convergent validity. With the exception of some age-based and functional groups, FOP-PFQ scores were significantly worse in groups with more severe disease, demonstrating known-groups validity.

Conclusion:​ The FOP-PFQ was demonstrated to be a reliable, valid measure that may be responsive to change in individuals with FOP, although some results were inconclusive for pediatric versions.