Discriminant validity of the Western Ontario and McMaster Universities Osteoarthritis index physical functioning subscale in community samples with hip osteoarthritis.

UNLABELLED: Pua Y-H, Cowan SM, Wrigley TV, Bennell KL. Discriminant validity of the Western Ontario and McMaster Universities Osteoarthritis Index Physical Functioning Subscale in community samples with hip osteoarthritis.
OBJECTIVE: To evaluate, in a community hip osteoarthritis (OA) sample, the discriminant validity of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) physical functioning (PF) subscale to differentiate between self-report measures of pain and physical function.
DESIGN: Cross-sectional.
SETTING: Human movement laboratory of a university. PARTICIPANTS: Adults (N=100; 60 women, 40 men; age, 62.3+/-10.1y) with radiographically confirmed symptomatic hip OA.
INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Six self-report measures of pain and physical function-WOMAC-PF and WOMAC-Pain subscales, Lower Extremity Functional Scale, Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36) bodily pain and PF subscales, and 4-item pain intensity measure-were obtained. Confirmatory factor analysis was applied to a correlated 2-factor measurement model that assumed discriminant validity: self-report measures of pain were conceptualized to load uniquely on 1 factor; self-report measures of physical function were conceptualized to load uniquely on the other factor.
RESULTS: Confirmatory factor analysis revealed that the initially proposed model did not achieve an acceptable fit to the data. Allowing a correlation between the error terms of the WOMAC-PF with those of the WOMAC-Pain and the SF-36 bodily pain subscales resulted in a viable model that provided adequate fit to the data (chi(2)=7.5, P=.28).
CONCLUSIONS: The findings suggest that the discriminant validity of the WOMAC-PF subscale from self-report pain measures cannot be confirmed in community-dwelling adults with hip OA.