Assessment of perceived functional capacity: Using Rasch analysis to evaluate the measurement properties of four perceived pain & disability scales.

Determination of readiness to return to work often involves having the individual participate in a Functional Capacity Evaluation (FCE). In this evaluation, scores on self-report measures are compared with scores on performance-based measures. A difference in the two scores can be interpreted as symptom magnification which can impact eligibility for benefits. FCEs include four widely used self report measures the Oswestry Disability Index (ODI), the Dallas Pain Questionnaire (DPQ), the Spinal Function Sort (SFS) and the Neck Disability Index (NDI). My objective was to use Rasch Modeling to evaluate the original classification categories of the ODI, DPQ, SFS and NDI to examine the hypothesis that the items in each scale form a unidimensional construct and to ascertain if the categories in the items are ordered to assess perceived disability. Data from 298 subjects presenting for FCE were fit to the Rasch Model. Examination included fit of data to model expectations, threshold ordering of items, differential item functioning and item difficulty. None of the scales demonstrated unidimensionality in their original format. For the ODI and DPQ, rescaling and/or eliminating items improved the scales. The SFS is not a unidimensional scale and demonstrates differential item functioning. The NDI does demonstrate unidimensionality when two of the items are eliminated but disordered thresholds could not be fixed. Health Professionals using these measures should be aware that they do not perform as well as expected. --P.ii.