COMPARISON OF A GENERIC AND A DISEASE-SPECIFIC MEASURE OF PAIN AND PHYSICAL FUNCTION AFTER KNEE REPLACEMENT SURGERY

Generic and disease-specific health status instruments are commonly us ed to assess patients' outcomes. The hypothesis that they measure dist inct but complementary aspects of patients' quality of life was tested using a sample of patients aged 67 to 99 years who had undergone knee replacement surgery 2 to 7 years previously. Patients' scores on a ge neric health-related quality-of-life (HRQOL) measure, the SF-36, were compared to those of the Western Ontario and McMaster Universities (WO MAC) Osteoarthritis Index; the WOMAC was developed specifically for pa tients with lower extremity arthritis, whereas the SF-36 is aimed at a ll conditions. A stratified sample of 1,750 Medicare beneficiaries was surveyed and an overall response rate of 80.3% achieved, resulting in 1;193 usable surveys (after adjustment for ineligible, incapacitated and deceased individuals). The distribution of scores on the three dim ensions common to both instruments (i.e., pain, physical function, and overall score) showed consistently higher scores on the WOMAC, on a s cale of 0 (worst) to 100 (best), than on the SF-36, indicating less di sability from arthritis than from other conditions after knee surgery in this elderly population. Statistically significant differences in t he number of people with perfectly healthy scores were detected betwee n the instruments; with regard to pain, 32.2% of the sample reported n o pain due to arthritis on the WOMAC, compared with only 13.6% reporti ng no pain due to any conditions on the SF-36. The figures for physica l function and overall score were 9.6% versus 1.4%, and 6.9% versus 0. 2%, respectively Examination for discriminant validity showed that the scores on the two scales followed hypothesized patterns: the WOMAC di scriminated better among subjects with varying severity of knee proble ms, whereas the SF-36 discriminated better among subjects with varying levels of self-reported health status and comorbidity. The results of this study support the inclusion of both a generic and a disease-spec ific HRQOL measure to assess patient outcomes fully.