title: An assessment of 25-hydroxyvitamin D measurements in comparability studies conducted by the Vitamin D Metabolites Quality Assurance Program.
abstract: BACKGROUND
  The National Institute of Standards and Technology (NIST), in collaboration with the National Institutes of Health Office of Dietary Supplements, established the first accuracy-based program for improving the comparability of vitamin D metabolite measurements, the Vitamin D Metabolites Quality Assurance Program.
  
  
  METHODS
  The study samples were human serum or plasma Standard Reference Materials (SRMs) with 25-hydroxyvitamin D values that were determined at NIST. Participants evaluated the materials using immunoassay (IA), liquid chromatography (LC) with mass spectrometric detection, and LC with ultraviolet absorbance detection. NIST evaluated the results for concordance within the participant community as well as trueness relative to the NIST value.
  
  
  RESULTS
  For the study materials that contain mostly 25-hydroxyvitamin D3 (25(OH)D3),the coefficient of variation (CV) for the participant results was consistently in the range from 7% to 19%, and the median values were biased high relative to the NIST values. However, for materials that contain significant concentrations of both 25-hydroxyvitamin D2 (25(OH)D2) and 25(OH)D3, the median IA results were biased lower than both the LC and the NIST values, and the CV was as high as 28%. The first interlaboratory comparison results for SRM 972a Vitamin D Metabolites in Human Serum are also reported.
  
  
  CONCLUSIONS
  Relatively large within-lab and between-lab variability hinders conclusive assessments of bias and accuracy.
authors: National Institute of Standards and Technology, Material Measurement Laboratory, Chemical Sciences Division, United States. Electronic address: vitdqap@nist.gov.
authors: Mary
authors: M
authors: Bedner
authors: Katrice A
authors: KA
authors: Lippa
authors: Susan S-C
authors: SS
authors: Tai
mesh_terms: D006801:Humans; D011785:Quality Assurance, Health Care; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood / Q000378:metabolism*
chemical_list: D014807:Vitamin D; C104450:25-hydroxyvitamin D
keywords: 25(OH)D(2); 25(OH)D(3); 25(OH)D(Total); 25-Hydroxyvitamin D; 25-hydroxyvitamin D(2); 25-hydroxyvitamin D(3); 3-epi-25(OH)D(3); 3-epi-25-hydroxyvitamin D(3); 95% confidence limit; ABVD; ALTM; Accuracy-Based Vitamin D; CLIA; DEQAS; EIA; IA; IOM; Immunoassay; Institute of Medicine; Interlaboratory comparison; Liquid chromatography; MAD(e); Mass spectrometry; NIH-ODS; NIST; NIST/NIH Vitamin D Metabolites Quality Assurance Program; National Institute of Standards and Technology; National Institutes of Health Office of Dietary Supplements; Quality assurance; RMP; SRM; Standard Reference Material; U(95); VitDQAP; Vitamin D External Quality Assessment Scheme; all-laboratory trimmed mean; chemiluminescence immunoassay; enzyme immunoassay; immunoassay; median absolute deviation estimate; reference measurement procedure; total 25-hydroxyvitamin D
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