Comparison of a generic (SF-36) and a disease specific (WOMAC) (Western Ontario and McMaster Universities Osteoarthritis Index) instrument in the measurement of outcomes after knee replacement surgery.
The discriminant validity of a generic, health related quality of life (HRQL) measure, the SF-36, was compared with that of a disease specific HRQL measure, the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), in patients aged 67-99 years who had undergone knee replacement surgery 2 to 7 years previously. A stratified random sample of 1750 Medicare beneficiaries was surveyed and 1193 usable responses were obtained (after adjustment for ineligible, incapacitated, and deceased individuals). Discriminant validity of scale scores common to both instruments (pain, physical functioning, and overall score) were compared. Examination of discriminant validity, using Spearman correlations, showed that the WOMAC discriminates better among individuals with knee problems, while the SF-36 discriminates better among individuals with varying levels of self reported general health status and comorbidities. These scales measure 2 distinct but important aspects of patients' health. These results support inclusion of both a generic and a disease specific HRQL measure in cross sectional studies.