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 (r = 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.