Qualitative Research to Explore the Patient Experience of X-Linked Hypophosphatemia and Evaluate the Suitability of the BPI-SF and WOMAC® as Clinical Trial End Points

X-linked hypophosphatemia (XLH) is a rare genetic disorder characterized by renal phosphate wasting and defective bone mineralization. Symptoms include bone pain, joint pain, stiffness, and fatigue. Published evidence regarding the patient experience of XLH is sparse and no XLH-specific outcome measures have been validated. To understand the symptoms, impacts, and patient experience of XLH and to evaluate the face and content validity of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC®) and the Brief Pain Inventory Short Form (BPI-SF) for use as end points in XLH clinical trials. Face-to-face, qualitative, semistructured interviews were conducted with 18 adults with XLH in the United States using concept elicitation and cognitive debriefing techniques. Open-ended questioning elicited spontaneous concepts focusing on XLH-associated symptoms and functional limitations. Cognitive debriefing of the WOMAC® and BPI-SF assessed the relevance and patient understanding of item wording, recall period, and response options. Various distinct symptom concepts were elicited including pain symptoms, dental symptoms, sensory symptoms, tiredness/fatigue symptoms, and musculoskeletal symptoms. Participants reported experiencing significant bone and joint pain, stiffness, mobility limitations, and an impact on their ability to work. Cognitive interviewing found both instruments to be relevant and well understood by most patients. The interviews generated rich, qualitative insights into the patient experience of XLH. Cognitive debriefing of the BPI-SF and WOMAC® supported their value as XLH clinical trial end points. Future research will assess the psychometric properties of these instruments for use in the XLH population.