Extension from inpatients to outpatients: validity and reliability of the Oxford Knee Score in measuring health outcomes in patients with knee osteoarthritis

Objective: To validate the Oxford Knee Score (OKS) in outpatients with knee osteoarthritis (OA). Methods: Eligible patients were interviewed using a pretested questionnaire containing the OKS, the Short Form (SF)-6D, and the EuroQol Group 5-Dimension Self-Report Questionnaire score (EQ-5D). Reliability was assessed using Cronbach's alpha, dimensionality using principal component factor analysis and item-total correlations, convergent and discriminant construct validity using expected correlations between the OKS and the SF-6D and the EQ-5D. Results: The OKS were well accepted by patients in a pilot testing. When administered to a convenient sample of 187 patients with knee OA (mean age 64 years, 74% female, mean duration of OA 7.6 years), Cronbach's alpha exceeded 0.8 and factor analysis yielded two factors with eigenvalues > 1. Hypothesized item-total correlations (rho >= 0.4) were observed for all items. Convergent construct validity was supported by the presence of hypothesized moderate to strong correlations between the OKS and SF-6D physical functioning, role limitation, social functioning and pain, EQ-5D mobility, self-care, usual activities, and pain/discomfort, and mobility visual analogue scale (rho = 0.47-0.82). Discriminant construct validity was not satisfactory. The OKS correlated weakly with SF-6D vitality (rho = 0.35), but strongly with SF-6D mental health and EQ-5D anxiety/depression (rho = 0.51 and 0.41, respectively). Conclusion: The OKS has demonstrated good psychometric properties and thus can be considered a reliable and valid measurement for outpatients with OA.