To conduct a systematic review of the measurement properties of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) and to evaluate the quality of WOMAC measurement studies using COSMIN (Consensus-Based Standards for the Selection of Health Measurement Instruments) criteria.
A search was conducted in the MEDLINE, CINAHL, Embase, PsycINFO, Scopus, and SPORTDiscus databases through September 2013. Data that assessed the WOMAC measurement model, reliability, validity, respondent burden, and equivalence across methods of administration were extracted. Overall study quality was rated following COSMIN criteria.
A total of 76 articles from 22 countries were included. Internal consistency reliability was consistently high (≥0.90) for the function scale and acceptable (≥0.70) for the pain and stiffness scales. Test-retest reliability was acceptable. Score equivalence was demonstrated across paper and electronic methods of data collection. Floor and ceiling effects were low except for notable (24-38%) proportions of patients achieving the best possible scores on the pain and stiffness scales 1-23 years after arthroplasty. Five exploratory factor analyses did not support a measurement model in which the pain and function items were distinct. Correlations between the WOMAC pain and function scales were high (median 0.79). The WOMAC pain and function scales had similar correlations with other pain measures, and therefore the WOMAC pain scale did not show divergent validity. COSMIN criteria were not fully met in most studies.
The WOMAC scales were reliable, but its pain scale was highly related to physical function. Further research into joint-specific pain measures that have broader content validity is needed.