title: Standardization of misleading immunoassay based 25-hydroxyvitamin D levels with liquid chromatography tandem-mass spectrometry in a large cohort study.
abstract: BACKGROUND
  The interest in vitamin D measurement has strongly increased in recent years. The best indicator for circulating vitamin D levels is 25-hydroxy-vitamin D (25(OH)D) which is often measured by different immunoassays. We demonstrate problems in comparability of measures by different immunoassays and the need for standardization in the context of a large population-based cohort study.
  
  
  METHODS
  25(OH)D was measured with the immunoassays Diasorin Liaison in 2006 in 5,386 women and in the context of another project with IDS-iSYS in 4,199 men in 2009-2010 (when the Diasorin Liaison was no longer available in the version utilized in 2006). Standardization was performed by re-measuring of 25(OH)D levels in 97 men and 97 women with liquid chromatography tandem-mass spectrometry (LC-MS/MS) to obtain linear regression conversion equations.
  
  
  RESULTS
  Applying a 30 nmol/L cut-off value for vitamin D deficiency would have resulted in 48.3% of women and 12.1% of men with vitamin D deficiency ahead of standardization. The large gender difference was strongly attenuated after standardization of the assays with only 15.7% of women and 14.3% of men with vitamin D deficiency. Standardization on average increased the 25(OH)D levels by 10.3 nmol/L in women and decreased 25(OH)D levels by 2.9 nmol/L in men.
  
  
  CONCLUSIONS
  The standardization with LC-MS/MS revealed that much of the observed gender difference was only assay-driven and the extremely high proportion of 48.3% vitamin D deficient women proved to be an exaggeration of the old version of the Diasorin-Liaison immunoassay. Standardization of 25(OH)D immunoassay results by LC-MS/MS is recommended to improve their accuracy and comparability, provided the LC-MS/MS method itself is adequately validated and standardized.
authors.affiliation: Division of Clinical Epidemiology and Aging Research, German Cancer Research Center, Heidelberg, Germany. b.schoettker@dkfz.de
authors.forename: Ben
authors.initials: B
authors.lastname: Schöttker
authors.forename: Eugène H J M
authors.initials: EH
authors.lastname: Jansen
authors.forename: Ulrike
authors.initials: U
authors.lastname: Haug
authors.forename: Lutz
authors.initials: L
authors.lastname: Schomburg
authors.forename: Josef
authors.initials: J
authors.lastname: Köhrle
authors.forename: Hermann
authors.initials: H
authors.lastname: Brenner
mesh_terms: D000368:Aged; D002853:Chromatography, Liquid / Q000379:methods*; D015331:Cohort Studies; D005260:Female; D006801:Humans; D007118:Immunoassay / Q000379:methods* / Q000592:standards*; D008297:Male; D008875:Middle Aged; D015203:Reproducibility of Results; D053719:Tandem Mass Spectrometry / Q000379:methods*; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D014808:Vitamin D Deficiency / Q000097:blood / Q000175:diagnosis*
chemical_list: D014807:Vitamin D; C104450:25-hydroxyvitamin D