title: Intraindividual variation in plasma 25-hydroxyvitamin D measures 5 years apart among postmenopausal women.
abstract: BACKGROUND
  Current literature examining associations between vitamin D and chronic disease generally use a single assessment of 25-hydroxyvitamin D [25(OH)D], assuming the 25(OH)D concentration of an individual is consistent over time.
  
  
  METHODS
  We investigated the intraindividual variability between two measures of plasma 25(OH)D concentrations collected approximately five years apart (1997-2000 to 2002-2005) in 672 postmenopausal women participating in the Women's Health Initiative. Plasma 25(OH)D was assessed using the DiaSorin LIAISON® chemiluminescence immunoassay. The within-pair coefficient of variation (CV) was 4.9% using blinded quality control samples. Mean and SDs of 25(OH)D at the two time points were compared using a paired t test. An intraindividual CV and intraclass correlation coefficient (ICC) were used to assess intraindividual variability. A Spearman correlation coefficient (r) assessed the strength of the association between the two measures, and concordance in vitamin D status at two time points was compared.
  
  
  RESULTS
  Mean 25(OH)D concentrations (nmol/L) significantly increased over time from 60.0 (SD = 22.2) to 67.8 (SD = 22.2; P < 0.05). The CV was 24.6%, the ICC [95% confidence interval (CI)] was 0.59 (0.54-0.64), and the Spearman r was 0.61 (95% CI = 0.56-0.66). Greater concordance over five years was observed in participants with sufficient compared with deficient or inadequate baseline 25(OH)D concentrations (weighted kappa = 0.39). Reliability measures were moderately influenced by season of blood draw and vitamin D supplement use.
  
  
  CONCLUSIONS
  There is moderate intraindividual variation in 25(OH)D concentrations over approximately five years.
  
  
  CONCLUSIONS
  These data support the use of a one-time measure of blood 25(OH)D in prospective studies with ≤ five years of follow-up.
authors.affiliation: Department of Social and Preventive Medicine, University at Buffalo, The State University of New York, Buffalo, NY 14214-8001, USA. aemillen@buffalo.edu
authors.forename: Jennifer E
authors.initials: JE
authors.lastname: Meng
authors.forename: Kathleen M
authors.initials: KM
authors.lastname: Hovey
authors.forename: Jean
authors.initials: J
authors.lastname: Wactawski-Wende
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Andrews
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Lamonte
authors.forename: Ronald L
authors.initials: RL
authors.lastname: Horst
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Genco
authors.forename: Amy E
authors.initials: AE
authors.lastname: Millen
mesh_terms: D000368:Aged; D005260:Female; D006801:Humans; D008875:Middle Aged; D017698:Postmenopause / Q000097:blood*; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000032:analysis / Q000097:blood
chemical_list: D014807:Vitamin D; C104450:25-hydroxyvitamin D