Fibroblast growth factor 23 (FGF23) concentrations must be interpreted in conjunction with serum phosphate (phosphorus) measurements, as FGF23 will be elevated in other conditions that cause hyperphosphatemia in vivo. These include chronic kidney disease; severe catabolic states (eg, severe systemic illness, uncontrolled type I diabetes mellitus, and severe starvation); vitamin D toxicity; intravenous phosphate treatment and very high phosphate diets; advanced malignancy in particular with tumor lysis; crush or other significant muscle injury or destruction; fractures; and some endocrine disorders, in particular hypoparathyroidism and acromegaly. With the exception of kidney failure, FGF23 measurements will not contribute to diagnosis or patient management in these situations.      

                                                                                                                                       Do not interpret FGF23 concentrations as absolute evidence of the presence or the absence of tumor induced osteomalacia (TIO). Some patients with TIO may have FGF23 levels within the reference interval. It is thought that tumors in these individuals may be secreting different, and yet unidentified, phosphatonins. Therefore, if the clinical picture and general osteomalacia laboratory workup strongly suggest that the patient has TIO, a normal intact FGF23 level should not discourage tumor search or removal.                             

In rare cases, some individuals can develop antibodies to mouse or other animal antibodies (often referred to as human anti-mouse antibodies [HAMA] or heterophile antibodies), which may cause interference in some immunoassays. Caution should be used in interpretation of results, and the laboratory should be alerted if the result does not correlate with the clinical presentation                                                                                             

Patients receiving burosumab therapy may have prolonged elevations of intact FGF23 in serum following monoclonal antibody administration. Measurement of intact FGF23 is not recommended on these patients. Serum phosphate, alkaline phosphatase, and 1,25(OH)2D measurements should be considered for monitoring response to therapy.