Rasch Analysis of the Dermatology Life Quality Index Reveals Limited Application to Chinese Patients with Skin Disease.

The objective of this study was to examine the psychometric properties of the Chinese version of the Dermatology Life Quality Index (DLQI) and to assess the invariance of its items with respect to several patient parameters via Rasch analysis. Data were aggregated from 9,845 patients with various skin diseases across 9 hospitals in different regions of China. The response structure, local independence, and reliability of the DLQI scale were analysed in a partial credit model, and differential item functioning (DIF) across region, disease, sex, and age were assessed with a Mantel-Haenszel procedure. Although acceptable scale reliability (Person Separation Index=2.3) was obtained, several problems were revealed, including disordered response thresholds, misfitting items, DIF by geogra-phical region and disease, and mis-targeting patients with mild impairment regarding health-related quality of life (HRQL). In conclusion, the DLQI provides inadequate information on patients' impairments in HRQL, and the application of the DLQI in Chinese patients with skin disease is limited.