Validity and reliability of the Foot Function Index (FFI) questionnaire Brazilian-Portuguese version

PURPOSE To evaluate the validity and reliability of the Foot Function Index (FFI) in its Brazilian Portuguese version. METHODS The validity and reliability of the FFI were tested in 50 volunteers, with plantar fasciitis, metatarsalgia and chronic ankle sprain. The FFI validity process used the Short Form-36 (SF-36) and Foot and Ankle Outcome Score (FAOS) questionnaires. The correlation between FFI, SF-36 and FAOS was done using the Pearson's linear coefficient. The inter and intra-evaluator reliability was ascertained by means of the intraclass correlation coefficient (ICC) and the internal consistency by means of Cronbach's alpha coefficient. The scores were used to assess the standard error measurement (SEM), minimal detectable change (MDC) and ceiling floor and effects. RESULTS The validity process showed that there were correlations between FFI and the "pain" and "social aspects" subscales of SF-36 and all subscales of FAOS, except for "other symptoms". The Brazilian-Portuguese version of FFI showed excellent intra and interevaluator correlations, with an ICC range of 0.99-0.97 and score reliability that was considered highly satisfactory, with Cronbach's alpha range of 0.80-0.61. The SEMs for inter and intra-evaluator reliability were 1.32 and 1.08, respectively. The MDC was 2.42 (90 % confidence interval). No ceiling and floor effect were detected. CONCLUSIONS The Brazilian-Portuguese version of the FFI questionnaire was found to be a valid and reliable instrument for foot function evaluation, and can be used both in scientific settings and in clinical practice.