Content validity of the EQ-5D-5L with skin irritation and self-confidence bolt-ons in patients with atopic dermatitis: a qualitative think-aloud study

**Background:** The EQ-5D-5L is a widely used generic health-related quality of life (HRQoL) measure, but its content validity may be limited in specific conditions like atopic dermatitis (AD). Two bolt-on dimensions—skin irritation and self-confidence—were developed for psoriasis (EQ-PSO) and are available in nearly 50 languages. This study aimed to assess the content validity (relevance, comprehensiveness, comprehensibility) of the EQ-5D-5L with these two bolt-ons in adult AD patients.

**Methods:** Between May 2021 and February 2022, semi-structured face-to-face interviews were conducted with 20 adult AD patients (median age 25 years, 60% female) recruited from a university dermatology clinic in Hungary. Patients completed the EQ-5D-5L plus the two bolt-ons using a concurrent think-aloud protocol, followed by probing questions based on COSMIN standards. Thematic analysis was performed by two researchers. Disease severity was assessed using SCORAD (median score 32.6; 15% clear, 35% mild, 25% moderate, 25% severe).

**Key Results:** Patients identified 36 important aspects of their lives affected by AD, with itch being the most frequently reported (95%). The EQ-5D-5L with bolt-ons was considered easy to complete and comprehensive. Skin irritation was ranked as the most relevant dimension by 45% of patients, followed by pain/discomfort (40%) and anxiety/depression (25%). No patient considered skin irritation, self-confidence, or pain/discomfort as the least relevant dimension. Fifteen potential missing concepts were identified, but only social relationships (n=5, 25%) and judgement by others (n=4, 20%) were mentioned by more than one patient. A conceptual overlap between skin irritation and pain/discomfort was noted by 7 patients (35%). Most patients (50%) preferred a 1-week recall period over 'today'. The self-care dimension was misinterpreted by 60% of patients, who understood it as problems with clothing or cosmetics causing irritation rather than difficulty performing activities.

**Clinical Implications:** The EQ-5D-5L with skin irritation and self-confidence bolt-ons demonstrates good content validity in AD patients, supporting its use in this population. However, the potential missing concepts of social relationships and stigma, and the conceptual overlap between skin irritation and pain/discomfort, warrant further investigation. The misinterpretation of the self-care dimension suggests a need for clearer wording or additional examples. Quantitative validation studies are recommended to confirm measurement properties in AD.