OBJECTIVES: X-linked hypophosphatemia (XLH) is a rare, lifelong metabolic bone disease with high patient burden. Disease burden can extend beyond the patient, leading to ‘spillover’ effects whereby patients’ wider networks experience health-related quality of life impacts. XLH is often hereditary, thus family members may have XLH and experience their own impacts in addition to a spillover effect. This study aimed to describe and measure the spillover effect of XLH on carers and family members of adults with XLH.

METHODS: A mixed-methods study was conducted with informal carers and family members of adults with XLH in the United Kingdom (UK). Quantitative data included: (1) participants’ and care recipients’ demographic and clinical information; (2) the EQ-5D-5L; (3) the Work Productivity and Activity Impairment (WPAI) questionnaire. Qualitative data were collected via semi-structured interviews.

RESULTS: Twenty carers and family members participated (N=4 had an XLH diagnosis). The mean EQ-5D utility for the total sample was 0.658 (0.202 and 0.772 for participants with and without a personal diagnosis of XLH respectively). The WPAI revealed an overall work impairment of 28% and activity impairment of 42%. Qualitative findings highlight several areas of support provided by carers and family members and substantial carer-burden, including impacts to emotional wellbeing, physical health, social and daily activities, work, and finances. There was some incongruence between qualitative and quantitative findings. For instance, most carers without XLH reported ‘No problems’ with their usual activities on the EQ-5D (69%), however, almost all participants qualitatively described substantial impacts to their daily activities.

CONCLUSIONS: This study highlights the substantial burden related to providing support for an adult with XLH, particularly among carers with an XLH diagnosis. The incongruence between some elements of the quantitative and qualitative findings raises important methodological considerations about the measurement of burden and spillover effects in XLH and other disease areas.