Rasch analysis of the dermatology life quality index (DLQI) in patients with mild to moderate-severe psoriasis

Background: The Dermatology Life Quality Index (DLQI) is a valuable tool for assessing the quality of life in adult patients with psoriasis. Aims: To analyse the reliability and validity of the DLQI to measure the quality of life in patients with mild to moderatesevere psoriasis. Methods: This was a secondary validation study nested in a follow-up study. The Rasch-Andrich model was utilised to perform response function, item and person fit, differential item functioning, dimensionality, and reliability analyses. Results: A total of 1439 patients were analysed, 52.1% male, mean age of 48.7 years (SD 16.1). Psoriasis vulgaris was the phenotype in 43.1% of patients, and 86% had a mild Psoriasis Area Severity Index (PASI). Adequate adjustment of the response function and the items was observed in the best-fit sample, except for item 7 (work and study). The measure explained 60.9% of the variance and presented a reliability of 0.86. Differential item functioning was identified by age, with a relevant bias in the estimation for older adults. Item-person maps are provided. Limitations: This study was performed at a single centre, with most patients presenting a mild PASI score, limiting generalisation of the findings. Conclusion: The validity evidence favours the use of the DLQI in moderate-severe psoriasis. However, the instrument biases the estimate of older adults. This population group should consider a specific version of the instrument.