Development of a disease-specific version of the EQ-5D-5L for use in patients suffering from psoriasis: lessons learned from a feasibility study in the UK

The EuroQol five-dimensional (EQ-5D) questionnaire is a generic measure widely used for the assessment of health status. Research has suggested that it may be insensitive to the burdens associated with particular conditions. This study was designed to explore the feasibility of developing and valuing a disease-specific "bolt-on" version of the EQ-5D questionnaire for use in psoriasis.

A series of steps were undertaken to develop, test, and evaluate dimensions for a psoriasis-specific version of the EQ-5D questionnaire (hereafter referred to as the EQ-PSO questionnaire). Candidate dimensions were explored through a review of published literature, in-depth qualitative interviews with patients, and consultation with a clinical expert. A psychometric validation exercise was then undertaken to establish how well dimensions functioned. Two dimensions were selected for inclusion in a draft measure alongside the existing EQ-5D questionnaire dimensions: "skin irritation" and "self-confidence." Last, a time trade-off valuation exercise was conducted with 300 members of the UK general public to derive utilities for health states described by the measure.

The psychometric analyses indicated that the two new candidate dimensions captured additional variance over and above the existing five dimensions. Data from the valuation exercise were analyzed by using different models. A collapsed random effects model was put forward as a parsimonious and accurate approach. Based on this model, estimated utilities ranged from 0.98 ± 0.02 for state "1111111" to 0.03 ± 0.29 for state "5555555."

This study has developed the EQ-PSO questionnaire to support future psoriasis research and has informed the development of future bolt-on versions of the EQ-5D questionnaire.