A new look at the Western Ontario and McMaster Universities OsteoarthritisIndex using Rasch analysis

Objective. The Western Ontario and McMaster Universities Osteoarthritis Ind ex (WOMAC) is a valid and widely used instrument for the assessment of oste oarthritis patients. We examined whether applying Rasch analysis, a new ite m-response theory, supports its metric properties. Methods. The metric properties were examined in a cohort of patients with o steoarthritis of the hip or knee. In an exploratory approach, a validated G erman version of the WOMAC was analyzed using Rasch analysis for its unidim ensionality and construct definition. Results. One hundred fifty-eight patients were included in the study. The 3 WOMAC domains-pain, stiffness, and physical function-were unidimensional w hen examined separately. In an analysis including all items, the stiffness items did not fit the model. This indicates that they are distinct from the pain and function items. Item difficulty analyses revealed redundancy betw een corresponding pain and function items addressing the same task (e.g., " pain while standing upright" and "difficulty while standing"). Conclusion. Applying Rasch analysis to the 3 WOMAC domains confirms their u nidimensionality. However, the pain and function items seem to represent th e same construct. Thus, if may be possible to simplify these domains to avo id redundancy The impact of reducing items must nevertheless be examined.