Generic and condition-specific outcome measures for people with osteoarthritis of the knee

The aims of this study were to evaluate two condition-specific and two generic health status questionnaires for measuring health-related quality of life in patients with osteoarthritis (OA) of the knee, and to offer guidance to clinicians and researchers in choosing between them.

Patients were recruited from two settings: 118 from knee surgery waiting lists and 112 from rheumatology clinics. Four self-completion questionnaires [Western Ontario and McMaster University Osteoarthritis Index (WOMAC), Health Assessment Questionnaire (HAQ), Short Form-36 (SF-36) and Euroqol] were sent to subjects on two occasions 6 months apart. Construct validity, convergent validity, internal consistency and responsiveness were examined using primarily non-parametric methods.

All instruments proved satisfactory in terms of ease of use, acceptability to patients, internal consistency and reliability. In the surgical group, the OA-specific WOMAC performed better than the HAQ and the generic measures in terms of validity and responsiveness to change, whereas in the rheumatology group the SF-36 was more responsive.

WOMAC is the instrument of choice for evaluating the outcome of knee replacement surgery in OA. The SF-36 provides a more general insight into patients' health and may be more responsive to change than the WOMAC in a heterogeneous rheumatology clinic population. Researchers wishing to undertake an economic evaluation might consider the EQ-5D for a surgical, but not a rheumatology clinic group.