Objective: To assess the reliability and validity of the Western Ontario and McMaster University Osteoarthritis Index (WOMAC) as an outcome measure in Asian patients with knee or hip osteoarthritis (OA) in Singapore.
Design: The WOMAC was administered twice 7 days apart to 66 consecutive English-speaking Chinese, Malay or Indian inpatients and outpatients with knee or hip OA seen at a tertiary referral centre through a structured interview, which also assessed demographic and other characteristics. Internal consistency was assessed using Cronbach's alpha, reliability using Spearman's correlations, intraclass correlations and repeatability coefficients, and relationships between WOMAC domains and known determinants of function using Spearman's correlations and the Mann-Whitney U-test.
Results: The WOMAC showed good internal consistency (alpha=0.70 to 0.93) and good reliability, with intraclass correlations of 0.83 to 0.90 and mean test-retest score differences of 0.02 to 0.13 points (possible range 4 points). Results of Likert scoring assessment supported the validity of the WOMAC when interpreted in the context of the close association between pain and physical function. Eight of nine a priori hypotheses relating WOMAC Pain and Physical Function scores to known determinants of function were present, supporting construct validity of this scale.
Conclusion: The WOMAC is a valid and reliable outcome measure in Asian patients with OA in Singapore.