title: Assessment of tubular reabsorption of phosphate as a surrogate marker for phosphate regulation in chronic kidney disease.
abstract: OBJECTIVE
  Fibroblast growth factor 23 (FGF23) and soluble α-Klotho are emerging potential biomarkers of phosphorus and vitamin D metabolism which change in concentration in early chronic kidney disease (CKD) in order to maintain normal phosphorus levels. Tubular reabsorption of phosphate (TRP) has been commonly used to assess renal tubular phosphate transport. The aim of this study was to evaluate the usefulness of TRP as a surrogate marker of parameters of CKD-mineral bone disease (CKD-MBD) in CKD.
  
  
  METHODS
  A cross-sectional study was performed in 93 stable patients with predialysis CKD stage 1-5. In all patients, TRP, estimated glomerular filtration rate (eGFR), calcium, phosphate, intact parathyroid hormone (iPTH), 25-hydroxyvitamin D, serum FGF23 and urine soluble α-Klotho levels were measured.
  
  
  RESULTS
  As renal function declined, TRP significantly decreased (P < 0.001; r = 0.763) and both iPTH and serum FGF23 increased (P < 0.001; r = -0.598, P < 0.001; r = -0.453, respectively). The prevalence of hyperphosphatemia, secondary hyperparathyroidism, FGF23 excess and abnormal TRP increased progressively with declining eGFR. Although TRP level changed later than FGF23, abnormal levels of both TRP and FGF23 were observed earlier than changes in iPTH and serum phosphate. Decreased TRP was found to be independently associated with decreased eGFR and increased iPTH but was not associated with urine soluble α-Klotho or serum FGF23 level in multiple linear regression analysis.
  
  
  CONCLUSIONS
  TRP is a simple, useful and cost-saving surrogate marker of the assessment of altered mineral metabolism in CKD patients and can be used as an alternative to serum FGF23, especially for mild to moderate renal insufficiency.
authors.affiliation: Division of Nephrology, Department of Internal Medicine, Korea University College of Medicine, Korea University Guro Hospital, 148, Gurodong-ro, Guro-gu, Seoul, 152-703, Korea.
authors.forename: Yu Ah
authors.initials: YA
authors.lastname: Hong
authors.forename: Ji Hee
authors.initials: JH
authors.lastname: Lim
authors.forename: Min Young
authors.initials: MY
authors.lastname: Kim
authors.forename: Yaeni
authors.initials: Y
authors.lastname: Kim
authors.forename: Keun Suk
authors.initials: KS
authors.lastname: Yang
authors.forename: Byung Ha
authors.initials: BH
authors.lastname: Chung
authors.forename: Sungjin
authors.initials: S
authors.lastname: Chung
authors.forename: Bum Soon
authors.initials: BS
authors.lastname: Choi
authors.forename: Chul Woo
authors.initials: CW
authors.lastname: Yang
authors.forename: Yong-Soo
authors.initials: YS
authors.lastname: Kim
authors.forename: Yoon Sik
authors.initials: YS
authors.lastname: Chang
authors.forename: Cheol Whee
authors.initials: CW
authors.lastname: Park
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015415:Biomarkers; D001851:Bone Diseases, Metabolic / Q000209:etiology / Q000378:metabolism*; D002118:Calcium / Q000097:blood; D003430:Cross-Sectional Studies; D005260:Female; D000089703:Fibroblast Growth Factor-23; D005346:Fibroblast Growth Factors / Q000097:blood*; D005919:Glomerular Filtration Rate; D005966:Glucuronidase / Q000652:urine; D006801:Humans; D006962:Hyperparathyroidism, Secondary / Q000097:blood / Q000209:etiology; D054559:Hyperphosphatemia / Q000097:blood / Q000209:etiology; D000090265:Klotho Proteins; D008297:Male; D008875:Middle Aged; D010281:Parathyroid Hormone / Q000097:blood; D010710:Phosphates / Q000097:blood / Q000378:metabolism*; D051436:Renal Insufficiency, Chronic / Q000150:complications / Q000378:metabolism* / Q000503:physiopathology; D065608:Renal Reabsorption; D014807:Vitamin D / Q000031:analogs & derivatives / Q000097:blood; D055815:Young Adult
chemical_list: D015415:Biomarkers; C000717427:FGF23 protein, human; D010281:Parathyroid Hormone; D010710:Phosphates; D014807:Vitamin D; D005346:Fibroblast Growth Factors; D000089703:Fibroblast Growth Factor-23; C104450:25-hydroxyvitamin D; D005966:Glucuronidase; D000090265:Klotho Proteins; D002118:Calcium