This study aims to provide Health-related Quality of Life (HRQoL) data from children and adolescents with growing skeletons in the United Kingdom (UK), including those treated with burosumab or alternative XLH treatment, as part of an updated submission to the SMC in early 2023. This study will utilise data from a subset of UK sites already within the XLH Registry (including participating Scottish sites) and collect additional HRQoL data within these sites (that are otherwise not included in the wider XLH Registry protocol). The HRQoL data will enable the calculation of HRQoL to derive the HRQoL utility estimates in children and adolescents with growing skeletons for the RSS health states, hence addressing an area of uncertainty.

This is a multicentre, non-interventional HRQoL study of children and adolescents with growing skeletons using routinely collected XLH Registry data and prospectively collected HRQoL data. This study will aim to enrol approximately 50 patients from approximately 5 UK centres (currently participating in the XLH Registry) over a 6-month period. Each patient will be followed for up to 12 months after enrolment. As stipulated, the data collection should not require additional patient visits or data collection within the health service.

Three types of data will be used in this study:

1. Routinely collected data from the XLH Registry on patient demographics and clinical data, as well as PedsQL HRQoL data.
2. HRQoL data prospectively collected at enrolment, 6-month and 12-month follow-up by the patient and/or their parent / legal guardian.
3. RSS scores within the XLH Registry will be used where available, or RSS will be centrally calculated retrospectively from radiographs of the affected wrist and/or knee. All RSS calculations will be centrally reviewed.

Inclusion Criteria:

1. Aged ≥4 years and <18 years.

2. Enrolled in the XLH Registry via one of the participating UK centres.

3. Have at least one of the following:

   1. RSS calculated at enrolment during a routine clinic visit. OR
   2. Historical radiographs of the affected wrist and/or knee available in the medical chart within the 6 months prior to enrolment, to allow RSS to be centrally calculated retrospectively.

4. Receiving any XLH treatment (burosumab, oral phosphate and active vitamin D, or other XLH treatment) within 30 days prior to informed consent for this study

Exclusion Criteria:

1. Does not have the cognitive capacity to provide informed consent, or their legally designated representative (i.e. parent / legal guardian) does not have the cognitive capacity to provide informed consent.
2. Is not expected to have open growth plates for the duration of the study.
3. Currently participating in an interventional clinical trial. Participation in a Compassionate Use Programme, Pre-commercial Programme, or Investigator Initiated Study does not preclude a patient from participation in this study.

The study will aim to enrol approximately 50 patients from at least 5 UK centres (currently participating in the XLH Registry).

Subjects will be receiving any XLH treatment (burosumab, oral phosphate and active vitamin D or another XLH treatment) within 30 days or prior to informed consent.