Design and validation of a new scale to assess the functional ability in children with juvenile idiopathic arthritis (JIA).

OBJECTIVE The assessment of the functional ability is one of the items of the core set to define improvements in patients with JIA, CHAQ being the most used scale already validated in 32 countries. The aim of this study was to design and validate a new scale named CAPFUN (capacidad funcional = functional ability) to assess functional ability in children with JIA. METHODS This scale includes 20 items, 8 of upper limbs, 8 of lower limbs, 3 combined, and 1 of cervical spine, developed in two steps according with OMERACT. Each item is scored: 0 when it is impossible to be performed, 1 when it is performed incompletely or with difficulties, and 2 when it is well performed. Seventy three patients with JIA according to ILAR criteria were assessed: 25 boys (34.2%) and 48 girls (65.8%) whose aver-age age was 12.8 years (95% CI 11.8 - 13.8) and the time from disease onset was 5.02 years (95% CI 3.9 - 6.1). For validation purposes, it was applied to 91 healthy children and adolescents. In every patient, correlation with active joints count and functional class according to Steinbrocker was assessed and with CHAQ in 31 patients in this series. RESULTS The CAPFUN index obtained in all healthy children was 2. Patients' media CAPFUN index was 1.54 (95%CI 1.38 - 1.68). The CAPFUN index for Steinbrocker's class I was 1.84 +/- 1.8; for class II 1.60 +/- 1.5 and for class III 0.91 +/- 1 (F 24.1 p < 0.001). CAPFUN showed significant correlation with CHAQ (Spearman coefficient -0.79 p < 0.001), with active joints count (Spearman coefficient -0.72 p < 0.001) and with Steinbrocker functional classes (Spearman coefficient -0.69 p < 0.001). This scale showed a good internal reliability (alpha coefficient equal to 0.94), its construct validity is demonstrated by its good correlation with Steinbrocker's scale and with CHAQ. CONCLUSION CAPFUN is a new instrument in order to assess functional ability in children with JIA. This scale showed a good internal reliability. Construct validity is demonstrated by its high correlation with Steinbrocker's scale and with CHAQ. This study demonstrates the usefulness of CAPFUN for the assessment of functional ability in children with JIA.