Convergent and discriminant validity of a generic and a disease-specific instrument to measure quality of life in patients with skin disease

Skindex is a quality-of-life instrument for skin diseases. To determine its convergent validity and its advantage relative to a generic measure, we compared responses of 132 dermatology patients to Skindex and the Medical Outcomes Study 36-item Short-Form Survey (SF-36). We hypothesized that (i) correlations between similar scales would be strong but not redundant (r = 0.5-0.6), and (ii) Skindex scores would correlate more highly with responses about skin disease-related aspects of health, and SF-36 scores would correlate more highly with responses concerning general health. As measured by the SF-36, patients reported general health status similar to the normal population, and SF-36 scores did not correlate with dermatologists' judgments about the severity of skin disease. Correlations between the same scales of the two instruments were as hypothesized (range of r, 0.44-0.56), and patients with low, medium, or high responses to Skindex differed similarly in SF-36 scores. On the other hand, some patients who reported on the SF-36 that they were free of physical symptoms (37% of patients) or social effects (54%) on Skindex, reported such effects from their skin disease. Also, responses about skin-related health aspects correlated more highly with Skindex than SF-36 (for skin condition, mean r = 0.42 vs 0.28; for disfigurement, 0.38 vs 0.24). Conversely, responses concerning general health correlated more highly with SF-36 than Skindex (for self-reported health status, mean r = 0.28 vs 0.16; for co-morbidity, 0.48 vs 0.37). This study further supports the validity of Skindex and also suggests that both generic and disease-specific health status measures can contribute to the assessment of patients with skin diseases.