The present invention relates generally to methods of in vitro diagnostics, in particular the use of a compound selected from the group consisting of fibroblast growth factor 23 (FGF23), inorganic phosphorus (P), the product of inorganic phosphorus and total calcium (P×tCa), osteopontin (OPN) and parathyroid hormone (PTH) as biomarker. Said biomarkers can be used to monitor the modulation of fibroblast growth factor receptor (FGFR) kinase activity, in particular its inhibition, and/or the occurrence of secondary effects of FGFR inhibition. The invention further provides methods and kits relating to these uses.
