OBJECTIVES: TIO is a rare condition in which benign tumors secrete excess fibroblast growth factor 23 (FGF23), resulting in phosphate wasting and hypophosphatemia, compromising muscle and bone structure and impacting function and wellbeing. A valid and reliable measure is required to evaluate the impact of treatment on health-related quality of life (HRQL). This study evaluated the item/scale properties, reliability, validity, and sensitivity to change of the SF-36 in adults with TIO and established meaningful change thresholds.

METHODS: Data were from two Phase 2 studies that evaluated the FGF23-blocking antibody burosumab in adults with TIO – the largest prospective dataset to date (n=27; 48% female; age 33–73 years).

RESULTS: Item responses for the SF-36 V2 were well distributed across the scale, with slight floor and ceiling effects. Cronbach’s alpha was ≥0.70 for all eight domains. Item–total correlations were ≥0.40 except for “vigorous activity” and “tired” in the Physical Functioning domain. For stability according to the 6-minute walk test (6MWT), intraclass correlation coefficients for test–retest were ≥0.75 for four domains and ≥0.40 for the other three. Multi-trait analysis supported the relevance of the domain structure in TIO (item–scale correlations ≥0.40 for all items except vigorous activity), with good support for convergent validity with the Brief Fatigue Inventory, Brief Pain Inventory Short-Form, and 6MWT. For known groups validity, scores differed in a logical direction for the 6MWT but small samples hindered significance testing. There was some indication that the SF-36 detects change over time in TIO. The change thresholds determined were in line with thresholds provided in the SF-36 user manual. Analyses based on subgroups were exploratory because of small samples but indicated test–retest reliability, known groups validity, and responsiveness.

CONCLUSIONS: The SF-36 is an appropriate measure to evaluate the effects of treatment on HRQL in TIO studies.