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 measuring foot pain, disability, and activity restriction in patients with rheumatoid arthritis. For the purposes of orthopaedic studies in which one foot serves as an internal control, we assessed the side-to-side reliability of the seven-question Foot Function Index pain subscale. Thirty patients with rheumatoid arthritis completed visual analog scale pain questionnaires for both feet on two occasions 8 days apart. Internal reliability of the scale was high, with Cronbach's alphas ranging from 0.94 to 0.96, suggesting good left versus right discriminatory abilities. Principal component factor analysis segregated the questions into two large clusters containing predominantly either left or right foot items. Intraclass correlation coefficients were examined for test-retest reliability (separated by side) and for side-to-side reliability (separated by the day of test). The resultant intraclass correlation coefficients were nearly equivalent, ranging from 0.79 to 0.89. Generalizability analysis yielded similar results. Intraclass correlation coefficients and generalizability analysis demonstrate that the majority of variation is best explained by the differences within subjects or between subjects rather than by test-retest or side-to-side differences. We recommend the Foot Function Index as a reliable measurement scale for use in orthopaedic interventional trials.