Having reference values of method-dependent iFGF23 is fundamental as some guidelines, approved by the competent national authorities, specify that FGF23 can be dosed at starting during therapy with burosumab, anti-FGF23 antibody, in the X-linked hypophosphatemia (XLH) but report decision values (RI) specific to an assay method (53). This evidence concerns the gene FGF23 and X-linked hypophosphatemia.