Validation of a modified Thai version of the Western Ontario and McMaster (WOMAC) osteoarthritis index for knee osteoarthritis.

The aim of this study was to evaluate the psychometric properties (validity and reliability) of the Thai version of the Western Ontario and McMaster (WOMAC) index in knee osteoarthritis (OA) patients. After permission from the copyright holder of the WOMAC index was obtained, the questionnaire was translated into Thai, backtranslated, tested for face and content validities, and then modified. A convergent validity was evaluated in 114 patients with knee OA by comparing the scores of a modified Thai WOMAC index to those of the algofunctional Lequesne index. A test-retest reliability was evaluated in 89 patients who answered the modified Thai WOMAC index twice at 3-week intervals. The statistical analyses for the content validity were index of content validity (ICV), floor and ceiling effects, and skewness of distribution; for the convergent validity was Spearman's correlation; for the test-retest reliability were Spearman's correlation and intraclass correlation; and for the internal consistency was Chronbach's alpha. The Thai WOMAC index had face validity. The ICV of the content validity ranged from 0.25-1.00. Two items (F05 and F12 of the original 24-item WOMAC index) that had an ICV of 0 were removed from the modified Thai version. The modified 22-item Thai WOMAC index had convergent validity to the algofunctional Lequesne index in pain and function dimensions (Spearman's correlation coefficients were 0.66 and 0.69, respectively). The test-retest reliability had correlation coefficients ranging from 0.65 to 0.71. The internal consistency had an alpha ranging from 0.85 to 0.97. In conclusion, the modified Thai WOMAC index had acceptable psychometric properties for Thai patients with knee OA.