Validity and reliability of Swedish WOMAC osteoarthritis index: a self-administered disease-specific questionnaire (WOMAC) versus generic instruments (SF-36 and NHP).

We assessed whether a disease-specific, self-administered questionnaire could replace a generic instrument as an outcome tool after total hip replacement, and tested the validity and reliability of the Swedish WOMAC osteoarthritis index. 58 patients operated on with total hip arthroplasty 2-10 years ago were randomized to the study. All patients were asked to answer one disease-specific questionnaire (WOMAC osteoarthritis index) and two generic instruments (NUP and SF-36) in the same week. The procedure was repeated after 4 weeks. We tested content validity, construct validity, criterion validity, test-retest reliability and internal consistency reliability according to total score, domains and items. We found that both the generic instruments (NHP and SF-36) and the disease-specific test (WOMAC) had a high validity, i.e., measuring what they were supposed to measure (high validity). The questionnaires were also reproducible over time (high reliability). We suggest the Swedish WOMAC to be used after total hip arthroplasty in future studies.