Rasch validation of the prosthetic mobility questionnaire: A new outcome measure for assessing mobility in people with lower limb amputation

Objective: To create a new outcome measure of mobility in people with lower limb amputation, based on a pool of 14 items assessing prosthetic mobility, comprising the 12-item Prosthesis Evaluation Questionnaire (Mobility Section) plus 2 new items. Design: Cross-sectional study. Subjects: A total of 100 subjects (median age 58 years; 46 males; median body mass index 23.8) who had recently undergone lower limb amputation. Methods: Each patient completed the 14-item questionnaire twice: (i) at admission to the rehabilitation unit for prosthetic rehabilitation training; (ii) at 6-month follow-up after discharge. Results: After Rasch analysis, 2 items were deleted (one due to misfit, the other because showing large (> 0.30) positive correlation of residuals with two other items (local dependence). The remaining items fitted the Rasch model (internal construct validity), giving a new 12-item scale with a 5-level response format, the Prosthetic Mobility Questionnaire (PMQ), which demonstrated unidimensionality, lack of differential item functioning, and good reliability indices (person-separation reliability = 0.87; Cronbach's alpha 0.88). Conclusion: Although further studies are needed to increase confidence in clinical use of the PMQ, this new questionnaire appears to be a promising, psychometrically-sound patientreported outcome measure for assessment of mobility in subjects with lower limb amputation who use a prosthesis.