Evaluation of the psychometric properties of the Functional Ability Scale in children with juvenile idiopathic arthritis

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Background and objectiveJuvenile idiopathic arthritis (JIA) has a generalized impact on physical function, thus functional capacity (FC) is one of the main outcome measures in this population. FC can be evaluated using self-referential questionnaires and with observational tests such as the Functional Ability Scale (CAPFUN). The psychometric properties of this scale have not been tested yet.The objective of this study to evaluate the psychometric properties of the CAPFUN in children with JIA according to Consensus-based Standards for the selection of health Measurements Instruments (COSMIN). Material and methodsPsychometric properties measurement study. Children were admitted between January 2018 and November 2019. Inclusion criteria: age between 6 and 16 years, diagnosis of JIA, complete physical therapy evaluation, including: CAPFUN scale, C-HAQ questionnaire, Visual Numerical Scale of patient's self-perceived Functional Capacity (VNSFC), Global Rating of Change (GROC) scale and Steinbrocker Scale. Subjects who discontinued follow-up were excluded. The psychometric properties evaluated were: reliability, construct validity, responsiveness, and interpretability. ResultsA total of 114 children were analyzed. The internal consistency was acceptable, as indicated by a Cronbach alpha of 0.93. Test–retest reliability showed a high level of consistency with an intraclass correlation coefficient of 0.999 (95% CI: 0.998–0.999). Regarding structural validity, the exploratory factor analysis yielded acceptable results, demonstrating the reliability of the instrument. All correlation coefficients were strong and all predetermined hypotheses were verified. The minimal clinically important difference was 0.04. [AUC 0.90 (95% CI: 0.83–0.91)]. ConclusionsThe evaluations of the psychometric properties of the CAPFUN were satisfactory, demonstrating acceptable values for reliability, construct validity, responsiveness, and interpretability. Based on these results, this tool can effectively assess FC in children with JIA, both in clinical practice and research contexts.