Intact Fibroblast Growth Factor 23 (Sendout)

Useful For:

• Diagnosing and monitoring tumor induced osteomalacia
• Diagnosing X-linked hypophosphatemia or autosomal dominant hypophosphatemic rickets
• Diagnosing familial tumoral calcinosis with hyperphosphatemia

Reference Values:

Cautions:

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: renal failure, 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 renal 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 suggest strongly that the patient has TIO; a normal intact FGF23 level should not discourage tumor search or removal.

Some patients who have been exposed to animal antigens, either in the environment or as part of treatment or imaging procedures, may have circulating anti-animal antibodies present. These antibodies may interfere with the assay reagents to produce unreliable results. Whenever the test results do not fit the clinical picture, the laboratory should be consulted regarding possible assay interference.

In vitro studies indicate that the presence of burosumab falsely decreases IFG23 results in a dose-dependent manner.

Patients receiving burosumab should not be monitored using the IFG23 assay. Serum phosphate, alkaline phosphatase, and 1,25(OH)2D measurements should be considered for monitoring response to therapy.

Outside Laboratories: Centrifuge sample and transfer serum to a separate plastic vial. Refrigerate serum.

Stability: Refrigerated (preferred): 14 days; Frozen: 90 days; Ambient: Unacceptable.

Reject Due To: Gross hemolysis. Gross lipemia okay.

Centrifuge sample and transfer serum to a separate plastic vial. Refrigerate serum.

Sendouts, order Mayo Test: IFG23.

Stability: Refrigerated (preferred): 14 days; Frozen: 90 days; Ambient: Unacceptable.

Reject Due To: Gross hemolysis. Gross lipemia okay.