Improved discriminative and evaluative capability of a refined version of Skindex, a quality-of-life instrument for patients with skin diseases

Objective: To improve Skindex, a dermatologic quality-of-life instrument. Design: Cross-sectional and longitudinal questionnaire study. Setting: Dermatology clinic of a Veterans Affairs hospital and private dermatology practices. Patients: Patients waiting for dermatology appointments; 201 patients responded to the original version of Skindex and 692 additional patients to the revised version. Main Outcome Measures: Reproducibility, internal consistency reliability, and validity of the revised version of Skindex. The revised version was compared with the original in 3 ways: the amount of time patients need to complete it; discriminative capability, determined as the number of items to which patients chose the same response; and evaluative capability, determined as the number of scales that were responsive to patients' reports of clinical change. Results: With the revised 29-item version of Skindex, scale scores were reproducible after 72 hours (v=0.88-0.92) and were internally reliable (Cronbach alpha=0.87-0.96). The instrument demonstrated both construct and content validity: patients with psoriasis and eczema responded with higher scores than those with isolated lesions; in an exploratory principal axes factor analysis with an oblique rotation, 97% of the common variance was explained by 3 factors that correlated with the a priori scales; and most patients' responses to an open-ended question about their skin disease were addressed by items in the instrument. The average time to complete the revised instrument was 5 minutes (compared with 15 minutes for the original version). For only 3 items (10%) did 70% or more of patients choose the same response (vs 17 [28%] of items in the original version). All scales changed significantly in the expected direction in patients who reported that their skin had changed after 3 months (vs only 3 of 8 scales originally). Conclusion: The 29-item version of Skindex remains reliable and valid, but has decreased respondent burden and improved discriminative and evaluative capability.