title: Significance of serum 24,25-dihydroxyvitamin D in the assessment of vitamin D status: a double-edged sword?
abstract: BACKGROUND
  24,25-Dihydroxyvitamin D [24,25(OH)2D] in serum may be both a nuisance and nutritionally valuable.
  
  
  METHODS
  We investigated the impact of 24,25(OH)2D3 on the performance of commercially available immunoassays for serum total 25-hydroxyvitamin D [25(OH)D] using (a) serum from a nationally representative sample of adults, (b) serum from a spiking experiment, and (c) data from the UK Vitamin D External Quality Assurance Scheme (DEQAS). We also investigated the utility of the serum ratio of 24,25(OH)2D3 to 25(OH)D as an index of inactivation and of response to vitamin D supplementation using randomized controlled trial (RCT) data. Measurement of 24,25(OH)2D in sera by a LC-MS/MS method allowed for an investigation of its impact on immunoassay-derived serum 25(OH)D values as well as its clinical utility. We report data from a nationally representative sample of adults, a recent vitamin D RCT in older adults, and DEQAS.
  
  
  RESULTS
  24,25(OH)2D3 contributed to the positive bias observed in some immunoassays relative to LC-MS/MS-derived estimates for total 25(OH)D. A spiking experiment showed that the degree of cross-reactivity with 24,25(OH)2D was high and may underpin this positive bias. Adjustment for 24,25(OH)2D3 concentration brought estimates closer to true values. Data from the vitamin D RCT showed that the ratio of 24,25(OH)2D3 to 25(OH)D was associated with serum 25(OH)D3 and with response of serum 25(OH)D to vitamin D supplementation.
  
  
  CONCLUSIONS
  Our findings highlight that the effect of 24,25(OH)2D3 in serum is a double-edged sword-an interferent for some immunoassays, yet potentially informative of nutritional status.
authors.affiliation: Vitamin D Research Group, School of Food and Nutritional Sciences, and Department of Medicine, University College Cork, Cork, Ireland; k.cashman@ucc.ie.
authors.forename: Kevin D
authors.initials: KD
authors.lastname: Cashman
authors.affiliation: Vitamin D Research Group, School of Food and Nutritional Sciences, and.
authors.forename: Aoife
authors.initials: A
authors.lastname: Hayes
authors.affiliation: Vitamin D Research Group, School of Food and Nutritional Sciences, and.
authors.forename: Karen
authors.initials: K
authors.lastname: Galvin
authors.affiliation: Office of Dietary Supplements, NIH, Bethesda, MD;
authors.forename: Joyce
authors.initials: J
authors.lastname: Merkel
authors.affiliation: Department of Biomedical and Molecular Sciences, Queen's University, Kingston, Ontario, Canada;
authors.forename: Glenville
authors.initials: G
authors.lastname: Jones
authors.affiliation: Department of Biomedical and Molecular Sciences, Queen's University, Kingston, Ontario, Canada;
authors.forename: Martin
authors.initials: M
authors.lastname: Kaufmann
authors.affiliation: Department of Laboratory Medicine, University of Washington, Seattle, WA;
authors.forename: Andrew N
authors.initials: AN
authors.lastname: Hoofnagle
authors.affiliation: Vitamin D External Quality Assurance Scheme (DEQAS) Coordination Centre, Imperial College, London, UK;
authors.forename: Graham D
authors.initials: GD
authors.lastname: Carter
authors.affiliation: Department of Public Health Sciences, Loyola University Stritch School of Medicine, Chicago, IL.
authors.forename: Ramon A
authors.initials: RA
authors.lastname: Durazo-Arvizu
authors.affiliation: Office of Dietary Supplements, NIH, Bethesda, MD;
authors.forename: Christopher T
authors.initials: CT
authors.lastname: Sempos
mesh_terms: D015650:24,25-Dihydroxyvitamin D 3 / Q000097:blood*; D002853:Chromatography, Liquid; D003429:Cross Reactions; D019587:Dietary Supplements; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D007124:Immunoenzyme Techniques / Q000592:standards*; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D053719:Tandem Mass Spectrometry; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood
chemical_list: D014807:Vitamin D; D015650:24,25-Dihydroxyvitamin D 3; C104450:25-hydroxyvitamin D