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 causing pain and fatigue. A valid and reliable patient-reported outcome (PRO) measure is required to evaluate the impact of treatment on TIO-associated pain and fatigue. This study evaluated the item/scale properties, reliability, validity, and sensitivity to change of the BFI and BPI-SF in adults with TIO and analyzed meaningful change thresholds.

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

RESULTS: The BFI and BPI-SF item responses were well distributed across the scale but with slight floor effects. Both instruments had high Cronbach’s alphas (0.972–0.955 [BFI] and 0.949–0.954 [BPI-SF]) suggesting item redundancy. Test–retest reliability was adequate based on stability according to the 6-minute walk test (6MWT) (intraclass correlation coefficients 0.505–0.753 and 0.724–0.809, respectively). Multi-trait analysis supported the relevance of each instrument’s domain structure in TIO (item–scale correlations ≥0.77 for BFI, ≥0.65 for BPI-SF). Both instruments had good convergent validity with each other, and with the Short-Form 36 V2. For known groups validity, scores differed in a logical direction for the 6MWT and sit-to-stand test, but small samples hindered significance testing. There was some indication that the BFI and BPI-SF detect change over time in TIO. Conservative estimates for meaningful change thresholds are 1.5 points for the BFI and 1.4 points for the BPI-SF (distribution-based method). Analysis based on subgroups was exploratory given small samples but indicated test–retest reliability, known groups validity, and responsiveness.

CONCLUSIONS: The BFI and BPI-SF are appropriate measures to evaluate the effects of treatment on pain and fatigue in TIO studies.