title: Estimating mean annual 25-hydroxyvitamin D concentrations from single measurements: the Multi-Ethnic Study of Atherosclerosis.
abstract: BACKGROUND
  The seasonal variation in circulating 25-hydroxyvitamin D [25(OH)D] concentrations is large relative to mean values. Single measurements may misclassify annual exposure, which may lead to bias in research and complicate clinical decision making.
  
  
  OBJECTIVE
  We aimed to develop and validate a model for adjusting a single measurement of a serum 25(OH)D concentration to the time of year it was measured.
  
  
  METHODS
  We measured serum 25(OH)D concentrations by using mass spectrometry in 6476 participants from the Multi-Ethnic Study of Atherosclerosis at baseline and again in a subset of 368 participants at a median of 17 mo later. We estimated a cosinor model to describe the seasonal variability in 25(OH)D concentrations and evaluated this model by using follow-up 25(OH)D measurements.
  
  
  RESULTS
  The mean age of subjects was 62.1 y, 61.2% of participants were nonwhite, and 53.3% of participants were women. The cosinor model predicted follow-up 25(OH)D concentrations better than a single measurement [difference in root mean squared error (RMSE): 1.3 ng/mL; P< 0.001]. The cosinor model also better predicted the measured annual mean 25(OH)D concentration (difference in RMSE: 1.0 ng/mL; P< 0.001). Annual mean 25(OH)D concentrations estimated from the cosinor model reclassified 7.1% of participants with regard to 25(OH)D deficiency, which was defined as <20 ng/mL. An estimated annual mean 25(OH)D concentration <20 ng/mL was significantly associated with lower bone mineral density, whereas an untransformed 25(OH)D concentration <20 ng/mL was not.
  
  
  CONCLUSIONS
  Cross-sectional data can be used to estimate subject-specific mean annual 25(OH)D concentrations from single values by using a cosinor model. The tool we developed by using this approach may assist research and clinical care of adults in North America by reducing the misclassification of 25(OH)D deficiency.
authors.affiliation: Kidney Research Institute and Division of Nephrology, University of Washington, Seattle, WA, USA. sachsmc@u.washington.edu
authors.forename: Michael C
authors.initials: MC
authors.lastname: Sachs
authors.forename: Abigail
authors.initials: A
authors.lastname: Shoben
authors.forename: Gregory P
authors.initials: GP
authors.lastname: Levin
authors.forename: Cassy
authors.initials: C
authors.lastname: Robinson-Cohen
authors.forename: Andrew N
authors.initials: AN
authors.lastname: Hoofnagle
authors.forename: Nancy
authors.initials: N
authors.lastname: Swords-Jenny
authors.forename: Joachim H
authors.initials: JH
authors.lastname: Ix
authors.forename: Matthew
authors.initials: M
authors.lastname: Budoff
authors.forename: Pamela L
authors.initials: PL
authors.lastname: Lutsey
authors.forename: David S
authors.initials: DS
authors.lastname: Siscovick
authors.forename: Bryan
authors.initials: B
authors.lastname: Kestenbaum
authors.forename: Ian H
authors.initials: IH
authors.lastname: de Boer
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D050197:Atherosclerosis / Q000150:complications / Q000503:physiopathology*; D015519:Bone Density; D003430:Cross-Sectional Studies; D005006:Ethnicity; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D015999:Multivariate Analysis; D012621:Seasons; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D014808:Vitamin D Deficiency / Q000097:blood* / Q000150:complications / Q000503:physiopathology
chemical_list: D014807:Vitamin D; C104450:25-hydroxyvitamin D