EXTENDED REPORTS Responsiveness of the WOMAC osteoarthritis index as compared with the SF-36 in patients with osteoarthritis of the legs undergoing a comprehensive rehabilitation intervention

Objective—To compare the responsiveness of the condition-specific Western Ontario and McMaster Universities osteoarthritis (OA) index (WOMAC) and the generic Short Form-36 (SF-36) in patients with OA of the legs undergoing a comprehensive inpatient rehabilitation intervention. Methods—A prospective follow up study of consecutively referred inpatients of a rehabilitation clinic was made. The patients included fulfilled the American College of Rheumatology criteria for knee or hip OA and underwent both passive and, particularly, active physical therapy for three to four weeks. Responsiveness assessment was performed using the standardised response mean (SRM), eVect size, and Guyatt’s responsiveness statistic between admission and discharge (end of rehabilitation) and then again between admission and three months later. For pain and function the SRMs were stratified by sex and OA joint. EVects were tested by the t test and SRMs of diVerent scales were compared by the jack knife test. Results—At the three month follow up, complete data were obtained for 223 patients. In general, the three responsiveness statistics showed a similar order of responsiveness. For both instruments, the pain scales were more responsive than the function scales. The responsiveness of the pain scale of both instruments was comparable (SRM=0.723 for WOMAC and SRM=0.528 for SF-36 at the end of rehabilitation; SRM=0.377 for WOMAC and SRM=0.468 for SF-36 at the three month follow up). In the measurement of function, the WOMAC was significantly more responsive than the SF-36 (SRMs, end of rehabilitation: 0.628 v 0.249; three month follow up: 0.235 v −0.001). Responsiveness tended to be higher in women and in knee OA than in men and hip OA. Conclusions—Both instruments, the WOMAC and the SF-36, capture improvement in pain in patients undergoing comprehensive inpatient rehabilitation intervention. Functional improvement can be detected better by the WOMAC than by the SF-36. All the other scales of both instruments were more weakly responsive. (Ann Rheum Dis 2001;60:834–840) Comprehensive assessment of patients with osteoarthritis (OA) of the legs includes both measurement of impairment—for example, symptoms and disability, and health related quality of life. Health related quality of life may be measured by condition-specific and generic health status questionnaires. 6 Disease-specific instruments are useful for measuring clinically important changes in response to treatments. Because these instruments include measurement of symptoms and abilities most relevant to a particular disease, they are usually better able to detect subtle improvements in health status than generic health status instruments. The most widely used condition-specific instruments for the assessment of hip or knee OA is the Western Ontario and McMaster Universities OA index (WOMAC), 4 8–13 which is recommended by the OMERACT (Outcome Measures in Rheumatology Clinical Trials). 15 General health status instruments measure multiple aspects of health, including, specifically, physical function, social function, and pain, and are suitable for comparison of health status between diseases. The most widely used generic instrument is the Short Form-36 (SF-36). 17 Validity and responsiveness are the most important criteria when deciding which particular instrument to use in a clinical trial or which to recommend to professional societies for inclusion in core sets, as exemplified by the