Cross-cultural inequivalence of dermatology-specific health-related quality of life instruments in psoriasis patients

The dermatology life questionnaire index ( DLQI) and the Skindex are the most commonly used dermatology-specific health- related quality of life ( HRQOL) instruments. Although these tools are used in international surveys and clinical trials, the cross- cultural equivalence of their items has not been documented. We used differential item functioning ( DIF), which is part of the Rasch model, to assess the impact of cultural background on the items of the DLQI and Skindex- 29 and- 17. The data of the 450 psoriasis patients, who attended in- and outpatient dermatology centers, was collected retrospectively from five European and one US center. The DLQI and Skindex- 29 scales did not fit the Rasch model ( P < 0.0008) and 10/ 10 of the DLQI and 19/ 29 of the Skindex- 29 items displayed significant DIF. Although the psychosocial scale of the Skindex- 17 fitted the Rasch model, half or more of the items of the psychosocial ( 6/ 12) and the symptom scale ( 4/ 5) showed significant DIF across countries. These findings suggest that psoriasis patients from different countries respond differently to a substantial proportion of DLQI and Skindex items despite having the same level of underlying HRQOL impairment. Therefore, these instruments should not be used in their current form in international studies.