Discriminant Validity of the Western Ontario and McMaster Universities Osteoarthritis Index Physical Functioning Subscale in Community Samples With Hip Osteoarthritis

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.ObjectiveTo 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.DesignCross-sectional.SettingHuman movement laboratory of a university.ParticipantsAdults (N=100; 60 women, 40 men; age, 62.3±10.1y) with radiographically confirmed symptomatic hip OA.InterventionsNot applicable.Main Outcome MeasuresSix 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.ResultsConfirmatory 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 (χ2=7.5, P=.28).ConclusionsThe 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.