The Burden of Adult X-Linked Hypophosphatemia on Carers and Family Members: A Mixed-Methods Study.

X-linked hypophosphatemia (XLH) is a rare, genetic disorder that severely impacts the health-related quality of life (HRQoL) of people living with the condition. This impact can also extend to carers and family members, described as a "spillover effect." Measurement of spillover effects can lead to greater understanding of disease burden and potentially to more equitable decision-making regarding adoption of treatments by health technology assessment (HTA) bodies. This study aimed to explore spillover effects among carers and family members of adults with XLH. This cross-sectional, mixed-methods study included carers and family members of adults diagnosed with XLH in the United Kingdom. Quantitative data included a background questionnaire, the EQ-5D-5L, and the Work Productivity and Activity Impairment Questionnaire (WPAI). Qualitative data were gathered through semi-structured interviews. Twenty carers or family members of adults with XLH completed the study. Four (20%) had an XLH diagnosis themselves. Mean (SD) EQ-5D utility was 0.66 (0.33) for the total sample, and 0.20 (0.31) and 0.77 (0.21) for participants with and without a personal diagnosis of XLH, respectively. The WPAI activity impairment was 42% and overall work impairment was 28%. Interviews revealed 6 areas of participants' lives impacted by XLH: emotional wellbeing, daily activities, work, finances, social lives and relationships, and physical impacts. Findings of this study indicate a substantial health-related quality-of-life and productivity burden for carers and family members of adults with XLH. The magnitude of this burden highlights the importance of incorporating such spillover effects in HTAs and broader rare disease policies.