THE FOOT FUNCTION INDEX FOR MEASURING RHEUMATOID-ARTHRITIS PAIN - EVALUATING SIDE-TO-SIDE RELIABILITY

The Foot Function Index is a validated and reliable instrument for mea suring foot pain, disability, and activity restriction in patients wit h rheumatoid arthritis. For the purposes of orthopaedic studies in whi ch one foot serves as an internal control, we assessed the side-to-sid e reliability of the seven-question Foot Function Index pain subscale. Thirty patients with rheumatoid arthritis completed visual analog sca le pain questionnaires for both feet on two occasions 8 days apart. In ternal reliability of the scale was high, with Cronbach's alphas rangi ng from 0.94 to 0.98, suggesting good left versus right discriminatory abilities. Principal component factor analysis segregated the questio ns into two large clusters containing predominately either left or rig ht foot items. Intraclass correlation coefficients were examined for t est-retest reliability (separated by side) and for side-to-side reliab ility (separated by the day of test). The resultant intraclass correla tion coefficients were nearly equivalent, ranging from 0.79 to 0.89, G eneralizability analysis yielded similar results. Intraclass correlati on coefficients and generalizability analysis demonstrate that the maj ority of variation is best explained by the differences within subject s or between subjects rather than by test-retest or side-to-side diffe rences. We recommend the Foot Function Index as a reliable measurement scale for use in orthopaedic interventional trials.