title: Soluble serum Klotho levels in healthy subjects. Comparison of two different immunoassays.
abstract: OBJECTIVE
  Soluble serum Klotho is a new biomarker linked to chronic kidney disease, cardiovascular disease and diabetes. This study describes the evaluation and comparison of two different immunoassays and establishment of assay specific reference intervals in adults.
  
  
  METHODS
  Serum Klotho concentrations were determined in 120 healthy adults aged 19-66 years. Blood samples were collected, and stored sera were assayed for Klotho according to age and gender. In addition several other clinical and laboratory characteristics were determined in the cohort and compared to the levels of serum Klotho.
  
  
  RESULTS
  Serum Klotho levels were significantly higher in time-resolved fluorescence immunoassay (TRF) compared to an ELISA (IBL) and no correlation was found between the assays. No signal was obtained in either assay when the standard curve was switched between the two different immunoassays. The median serum Klotho concentration using TRF was 61 ng/mL (2.5-97.5% reference limits; 11-181 ng/mL) for males and 99 ng/mL (2.5-97.5% reference limits; 19-316 ng/mL) for females while the ELISA gave a mean value of 472 pg/mL (2.5-97.5% reference limits; 204-741 pg/mL) with no difference between genders. Concentrations of serum Klotho were independently associated with estimated glomerular filtration rate (eGFR) and body weight using TRF whereas serum Klotho concentrations were associated with age using the ELISA.
  
  
  CONCLUSIONS
  Comparison of two different immunoassays for serum Klotho indicate, that the protein exists in human beings in different forms which may function as independent factors and whose role and potential value as biomarkers needs to be evaluated separately. Reference intervals specific for the different forms recognized by the different assays were calculated in this study.
authors.affiliation: Department of Clinical Biochemistry and Pharmacology, Odense University Hospital, Odense, Denmark. Electronic address: Lise-Pedersen@ouh.regionsyddanmark.dk.
authors.forename: Lise
authors.initials: L
authors.lastname: Pedersen
authors.affiliation: Department of Clinical Biochemistry and Pharmacology, Odense University Hospital, Odense, Denmark.
authors.forename: Susanne Møller
authors.initials: SM
authors.lastname: Pedersen
authors.affiliation: Department of Clinical Biochemistry, Vejle Hospital, Denmark.
authors.forename: Claus Lohman
authors.initials: CL
authors.lastname: Brasen
authors.affiliation: Department of Clinical Biochemistry and Pharmacology, Odense University Hospital, Odense, Denmark.
authors.forename: Lars Melholt
authors.initials: LM
authors.lastname: Rasmussen
mesh_terms: D000328:Adult; D000368:Aged; D004797:Enzyme-Linked Immunosorbent Assay; D005260:Female; D000089703:Fibroblast Growth Factor-23; D005346:Fibroblast Growth Factors / Q000097:blood; D005966:Glucuronidase / Q000097:blood*; D006262:Health; D006801:Humans; D007118:Immunoassay / Q000379:methods*; D000090265:Klotho Proteins; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D012044:Regression Analysis; D012995:Solubility; D018709:Statistics, Nonparametric; D055815:Young Adult
chemical_list: C000717427:FGF23 protein, human; D005346:Fibroblast Growth Factors; D000089703:Fibroblast Growth Factor-23; D005966:Glucuronidase; D000090265:Klotho Proteins
keywords: Age; BMI; FGF23; Immunoassays; Ionized calcium; Klotho; PTH; Phosphate; Reference ranges; eGFR