title: Variability and reproducibility of circulating vitamin D in a nationwide U.S. population.
abstract: BACKGROUND
  Most studies examining associations between circulating vitamin D and disease are based on a single measure of vitamin D, which may not reflect levels over time, particularly because vitamin D concentrations vary by season. Few studies evaluated how well multiple 25-hydroxyvitamin D [25(OH)D] measures track within the same individual over time.
  
  
  OBJECTIVE
  This study examined variability and reproducibility of vitamin D by evaluating repeat measurements of plasma 25(OH)D concentrations while accounting for determinants of circulating concentrations including dietary supplement use and latitude of residence from a population of U.S. radiologic technologists.
  
  
  METHODS
  We analyzed circulating 25(OH)D in blood samples taken from 538 men and women from a prospective, nationwide study at two time points within a 1-yr period, most measured in different seasons. Inter- and intra-individual variability, reliability coefficients, and measurement error were examined.
  
  
  RESULTS
  The spearman rank correlation between two measurements of 25(OH)D concentrations was moderate (r = 0.75, P < 0.001) and did not vary significantly by participant characteristics including age, race, or latitude. The intraclass correlation coefficient was 0.72 (95% confidence interval = 0.68-0.76). The deattenuation factor of plasma 25(OH)D levels was 1.39, suggesting that a single measure of vitamin D on a continuous scale in regression analyses may result in attenuated relationships of about 40%.
  
  
  CONCLUSIONS
  Our results suggest that a single blood sample obtained in spring or fall provides a reasonable average for 25(OH)D over a 1-yr period, but additional studies are needed to estimate variability and agreement in plasma 25(OH)D measurements over longer intervals and younger populations.
authors.affiliation: Division of Cancer Epidemiology and Genetics, National Cancer Institute, 6120 Executive Boulevard, Bethesda, Maryland 20852, USA. majorjm@mail.nih.gov
authors.forename: Jacqueline M
authors.initials: JM
authors.lastname: Major
authors.forename: Barry I
authors.initials: BI
authors.lastname: Graubard
authors.forename: Kevin W
authors.initials: KW
authors.lastname: Dodd
authors.forename: Allison
authors.initials: A
authors.lastname: Iwan
authors.forename: Bruce H
authors.initials: BH
authors.lastname: Alexander
authors.forename: Martha S
authors.initials: MS
authors.lastname: Linet
authors.forename: D Michal
authors.initials: DM
authors.lastname: Freedman
mesh_terms: D000368:Aged; D001774:Blood Chemical Analysis / Q000592:standards / Q000706:statistics & numerical data*; D003940:Diagnostic Techniques, Endocrine / Q000592:standards / Q000706:statistics & numerical data; D005260:Female; D006801:Humans; D007206:Individuality; D008297:Male; D008875:Middle Aged; D015588:Observer Variation; D009994:Osmolar Concentration; D011153:Population; D012016:Reference Values; D015203:Reproducibility of Results; D012621:Seasons; D014481:United States / Q000453:epidemiology; D014807:Vitamin D / Q000032:analysis / Q000097:blood*; D014808:Vitamin D Deficiency / Q000097:blood / Q000175:diagnosis / Q000453:epidemiology
chemical_list: D014807:Vitamin D