RESUMEN
PURPOSE: The Manchester Foot Pain and Disability Index (MFPDI) is a self-assessment 19-item questionnaire developed in the UK to measure foot pain and disability. This study aimed at conducting cross-cultural adaptation and validation of the MFPDI for use in Spain. METHODS: Principles of good practice for the translation and cultural adaptation process for patient-reported outcomes measures were followed in the MFPDI adaptation into Spanish. The cross-cultural validation involved Rasch analysis of pooled data sets from Spain and the UK. RESULTS: Spanish data set comprised 338 patients, five used in the adaptation phase and 333 in the cross-cultural validation phase, mean age (SD) = 55.2 (16.7) and 248 (74.5 %) were female. A UK data set (n = 682) added in the cross-cultural validation phase; mean age (SD) = 51.6 (15.2 %) and 416 (61.0 %) were female. A preliminary analysis of the 17-item MFPDI revealed significant local dependency of items causing significant deviation from the Rasch model. Grouping all items into testlets and re-analysing the MFPDI as a 3-testlet scale resulted in an adequate fit to the Rasch model, χ (2) (df) = 15.945 (12), p = 0.194, excellent reliability and unidimensionality. Lack of cross-cultural invariance was evident on the functional and personal appearance testlets. Splitting the affected testlets discounted the cross-cultural bias and satisfied requirements of the Rasch model. Subsequently, the MFPDI was calibrated into interval-level scales, fully adjusted to allow parametric analyses and cross-cultural data comparisons when required. CONCLUSIONS: Rasch analysis has confirmed that the MFPDI is a robust 3-subscale measure of foot pain, function and appearance in both its English and Spanish versions.