Fibroblast Growth Factor 23

CPT: 83520
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Test Details

Synonyms

  • FGF-23 C-Terminal Fragment

Use

Fibroblast growth factor 23 (FGF-23), a member of the fibroblast growth factor (FGF) family of proteins, is a phosphaturic hormone predominantly produced by bone osteocytes.1-6 Plasma FGF-23 exerts its actions by binding to the FGF receptors on cell membranes. Effective FGF-23 binding to these cell surface receptors requires that the cell membrane also contain the transmembrane protein, Klotho.1,7-10 FGF-23 inhibits phosphate reabsorption by suppressing Na/Pi cotransporter activity in the proximal convoluted tubule of the kidney. In addition, FGF-23 suppresses intestinal phosphate absorption by inhibiting 1-α-hydroxylase, the enzyme responsible for the conversion of calcifediol to calcitriol, the biologically active form of vitamin D.1-3 FGF-23 also possibly inhibits parathyroid hormone (PTH) synthesis and secretion.1-9 In healthy individuals, low levels of FGF-23 are detected in the circulation, and FGF-23 secretion rises with increased phosphorus intake and increased calcitriol levels.11

Elevated plasma FGF-23 activity has been associated with several hypophosphatemic diseases characterized by impaired renal phosphate reabsorption and rickets/osteomalacia.4,5,8 Mutations in FGF-23 that render the protein resistant to proteolytic cleavage lead to increased FGF-23 activity and the renal phosphate loss found in autosomal-dominant hypophosphatemic rickets (ADHR). X-linked hypophosphatemic rickets (XLH)4,5,8 and autosomal-recessive hypophosphatemic rickets (ARHR) are due to mutations in PHEX and dentin matrix protein 1, respectively. Both disorders are characterized by overproduction of FGF-23 by bone osteocytes. In tumor-induced osteomalacia (TIO), an acquired disorder of renal phosphate wasting associated with tumors, typically of mesenchymal origin; phosphatonins produced by the tumor promote renal phosphate wasting. FGF-23 is the most common phosphatonin found in patients with TIO.4 Patients with TIO share similar biochemical and skeletal phenotypes with patients who have ADHR, ARHR, and XLH.4

FGF-23 levels increase dramatically as renal function declines in chronic kidney disease (CKD) as the body attempts to overcome persistent phosphate retention.1,3 FGF-23 elevation is thought to play a role in causing the disordered bone and mineral metabolism seen in CKD patients.1,3 FGF-23 levels increase in parallel with the decline in renal function well before a significant increase in serum phosphate concentration or PTH occur.1,3 Increased FGF-23 levels lead to reduced renal production of 1,25-dihydroxyvitamin D and to hypersecretion of parathyroid hormone.2 Prospective studies have demonstrated that elevated FGF-23 levels predict faster disease progression in CKD patients not on dialysis and increased mortality in patients undergoing maintenance hemodialysis and patients with renal transplants.1,12 FGF-23 may predict future development of refractory hyperparathyroidism and cardiovascular events in CKD patients2,13 and is thought to play a central role in the pathogenesis of post-transplant hypophosphatemia in kidney transplant recipients.2

Limitations

Results of this test are labeled for research purposes only by the assay's manufacturer. The performance characteristics of this assay have not been established by the manufacturer. The result should not be used for treatment or for diagnostic purposes without confirmation of the diagnosis by another medically established diagnostic product or procedure. The performance characteristics were determined by LabCorp.

Methodology

Enzyme-linked immunosorbent assay (ELISA)14 This test is a second-generation C-terminal assay that measures both the intact FGF-23 and its C-terminal fragments.14

Specimen Requirements

Specimen

Plasma

Volume

1 mL

Minimum Volume

0.3 mL (Note: This volume does not allow for repeat testing.)

Container

Lavender-top (EDTA) tube

Patient Preparation

A morning 12-hour fasting sample is recommended but not required.

Collection

Separate plasma from cells within 60 minutes and transfer to a plastic transport tube.

Storage Instructions

Refrigerate.

Stability Requirements

Temperature

Period

Room temperature

16 hours

Refrigerated

14 days

Frozen

14 days

Freeze/thaw cycles

Stable x3

Causes for Rejection

Gross lipemia; nonplasma sample received; plasma sample received that is not EDTA

Clinical Information

Footnotes

1. Nitta K. Relationship between fibroblast growth factor-23 and mineral metabolism in chronic kidney disease. Int J Nephrol. 2010 Dec 30; 2010:167984. 21234310
2. Nakai K, Komaba H, Fukagawa M. New insights into the role of fibroblast growth factor 23 in chronic kidney disease. J Nephrol. 2010 Nov-Dec; 23(6):619-625. 20658451
3. Wesseling-Perry K. FGF-23 in bone biology. Pediatr Nephrol. 2010 Apr; 25(4):603-608. Wesseling-Perry K
4. Farrow EG, White KE. Tumor-induced osteomalacia. Expert Rev Endocrinol Metab. 2009 Sep 1; 4(5):435-442. 20228870
5. Saito T, Fukumoto S. Fibroblast growth factor 23 (FGF23) and disorders of phosphate metabolism. Int J Pediatr Endocrinol. 2009 Oct 7; 2009:496514. 19956747
6. Berndt T, Kumar R. Novel mechanisms in the regulation of phosphorus homeostasis. Physiology (Bethesda). 2009 Feb; 24:17-25. 19196648
7. Razzaque MS. FGF23-mediated regulation of systemic phosphate homeostasis: Is Klotho an essential player? Am J Physiol Renal Physiol. 2009 Mar; 296(3):F470-F476. 19019915
8. Fukumoto S. Physiological regulation and disorders of phosphate metabolism—pivotal role of fibroblast growth factor 23. Intern Med. 2008; 47(5):337-343. 18310961
9. Razzaque MS, Lanske B. The emerging role of the fibroblast growth factor-23-klotho axis in renal regulation of phosphate homeostasis. J Endocrinol. 2007 Jul; 194(1):1-10. 17592015
10. Kuro-O M. Phosphate and Klotho. Kidney Int Suppl. 2011 Apr; (121):S20-S23. 21346722
11. Stubbs J, Liu S, Quarles LD. Role of fibroblast growth factor 23 in phosphate homeostasis and pathogenesis of disordered mineral metabolism in chronic kidney disease. Semin Dial. 2007 Jul-Aug; 20(4):302-308. 17635819
12. Wolf M, Molnar MZ, Amaral AP, et al. Elevated FGF 23 Is a risk factor for kidney transplant loss and mortality. J Am Soc Nephrol. 2011 May; 22(5):956-966. 21436289
13. Seiler S, Reichart B, Roth D, Seibert E, Fliser D, Heine GH. FGF-23 and future cardiovascular events in patients with chronic kidney disease before initiation of dialysis treatment. Nephrol Dial Transplant. 2010 Dec; 25(12):3893-3899. 20525642
14. Human FGF-23 (C-Term) ELISA Kit, 2nd Generation Enzyme-Linked ImmunoSorbent Assay (ELISA) for the Determination of Human Fibroblast Growth Factor 23 Levels in Plasma or Cell Culture Media. [package insert] Immutopics Inc; January 2009. 90-6100.

LOINC® Map

Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
004380 FGF-23 46699-5 004383 FGF-23 RU/mL 46699-5

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