Qualitative testing of the EQ-HWB-9 with a ‘blank’ EQ-HWB VAS and EQ-5D-5L with skin bolt-ons in a rare genetic skin disease population

Epidermal differentiation disorders (EDDs) are rare genetic skin disorders that significantly impact patients’ health and wellbeing. This study assessed the content validity of two experimental EuroQol instruments (EQ-5D-5Lbolt-ons and EQ-HWB-9) and explored potential endpoint options for a visual analogue scale (VAS) for EQ-HWB-9 in this population. Semi-structured interviews were conducted with 20 adult patients with EDDs at a university dermatology clinic (2024–2025). Participants completed the EQ-5D-5L with skin irritation and self-confidence bolt-ons and EQ-HWB-9 using a think-aloud protocol. Probing questions explored item relevance, missing concepts, and comprehensibility. An EQ-HWB VAS with endpoints blanked out was also presented for feedback. Participants found both instruments generally comprehensive, understandable, and relevant for EDD. The EQ-5D-5L with bolt-ons was considered suitable for measuring health- and disease-related aspects (e.g., skin irritation bolt-on was the most relevant item for all but one patients), while the EQ-HWB-9 was seen as relevant to measure broader life impacts. Three participants suggested combining the EQ-HWB-9 loneliness item with others, and three participants recommended splitting the sadness/depression item. One participant noted missing social/romantic relationship items in both instruments. Overall, 32 different concepts were proposed as EQ-HWB VAS endpoint labels, including both health-related and broader wellbeing concepts, such as overall feeling, mood, quality of life, and harmony. Both the EQ-5D-5L with bolt-ons and EQ-HWB-9 demonstrated acceptable content validity in this genetic disease population. Our findings provide useful input for finalizing these experimental instruments and offer the first exploratory qualitative results about potential endpoints for an EQ-HWB VAS.