title: Correlation among 25-hydroxy-vitamin D assays.
abstract: BACKGROUND
  Measurement of circulating 25-hydroxy-vitamin D [25(OH)D]) is the accepted clinical indicator of vitamin D status. However, between-laboratory differences in measurement of this analyte exist, which may confound clinical care.
  
  
  OBJECTIVE
  We investigated the current agreement of 25(OH)D measurement in clinical laboratories and explored the possibility that simple calibration would improve between-laboratory agreement.
  
  
  METHODS
  Serum obtained from healthy volunteers (age 20-60 yr) and one "calibrator," selected to have a 25(OH)D value near 30 ng/ml, were sent for 25(OH)D measurement in four clinical laboratories (laboratories A-D) using HPLC, liquid chromatography tandem mass spectroscopy, and RIA methodologies.
  
  
  METHODS
  Serum 25(OH)D. Based upon self-report, the laboratory with the lowest interassay percent coefficient of variation was assigned as the reference to which the others were compared using linear regression and Bland-Altman analyses (Analyse-it; Analyse-it Software, Ltd., Leeds, UK).
  
  
  RESULTS
  Good correlation was observed for 25(OH)D measurement between laboratory A and laboratories B-D (R(2) = 0.99, 0.81, and 0.95, respectively). Modest between-laboratory variation was noted; the mean bias ranged from 2.9-5.2 ng/ml. Consistent with a systematic offset, each value in laboratory B was higher than in laboratory A, and 89% of values from laboratories B-D were higher than laboratory A. The use of a single calibrator and correction factor reduced mean between-laboratory bias for laboratories B and D.
  
  
  CONCLUSIONS
  Measurement of 25(OH)D by clinical laboratories yields similar results. The use of even a single calibrator will improve, but not resolve, between-laboratory variability. Based upon these data, in combination with reported within-individual variability, we recommend that clinicians aim for values greater than 30 ng/ml in their patients.
authors.affiliation: Osteoporosis Clinical Center and Research Program, University of Wisconsin, 2870 University Avenue, Madison, WI 53705, USA. nbinkley@wisc.edu
authors.forename: N
authors.initials: N
authors.lastname: Binkley
authors.forename: D
authors.initials: D
authors.lastname: Krueger
authors.forename: D
authors.initials: D
authors.lastname: Gemar
authors.forename: M K
authors.initials: MK
authors.lastname: Drezner
mesh_terms: D000328:Adult; D002138:Calibration; D002851:Chromatography, High Pressure Liquid; D005260:Female; D006801:Humans; D016014:Linear Models; D008297:Male; D013058:Mass Spectrometry; D008875:Middle Aged; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood
chemical_list: D014807:Vitamin D; C104450:25-hydroxyvitamin D