title: Comparison of commercially available (125)I-based RIA methods for the determination of circulating 25-hydroxyvitamin D.
abstract: BACKGROUND
  Measurement of circulating 25-hydroxyvitamin D [25(OH)D] is important in the management of metabolic bone disease. The aim of this study was to compare two widely used methods for the quantification of circulating 25(OH)D with attention to their abilities to measure 25-hydroxylated ergocalciferol (vitamin D(2)) [25(OH)D(2)] and cholecalciferol (vitamin D(3)) [25(OH)D(3)].
  
  
  METHODS
  We used two commercially available, Food and Drug Administration-approved, radioiodine ((125)I)-based RIA kits for the detection of 25(OH)D (DiaSorin, Stillwater, MN and IDS Ltd, Tyne and Wear, United Kingdom). These methods were tested for general assay performance, including antibody specificity. Results were compared with those of an HPLC-based direct ultraviolet detection method.
  
  
  RESULTS
  Within- and between-run CVs were </=10%. Both methods quantitatively recovered 25(OH)D(3) added to serum, but only the DiaSorin kit quantitatively recovered 25(OH)D(2). The primary antibody in the IDS kit had unequal reactivities with pure 25(OH)D(2) and 25(OH)D(3), whereas the DiaSorin primary antibody reacted with them equally. In 50 patient samples assayed by HPLC, the IDS method, but not the DiaSorin method, underestimated total circulating 25(OH)D when significant circulating 25(OH)D(2) was present in patient samples.
  
  
  CONCLUSIONS
  Some immunoassays may underestimate total 25(OH)D when 25(OH)D(2) constitutes an appreciable part of the total.
authors.affiliation: Departments of Pediatrics, Biochemistry and Molecular Biology, Medical University of South Carolina, 171 Ashley Ave., Charleston, SC 29425. Hollisb@musc.edu
authors.forename: B W
authors.initials: BW
authors.lastname: Hollis
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000918:Antibody Specificity; D002648:Child; D002675:Child, Preschool; D002762:Cholecalciferol / Q000097:blood; D002851:Chromatography, High Pressure Liquid; D004872:Ergocalciferols / Q000097:blood; D006801:Humans; D007223:Infant; D007457:Iodine Radioisotopes; D008875:Middle Aged; D011863:Radioimmunoassay; D012680:Sensitivity and Specificity; D013056:Spectrophotometry, Ultraviolet; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood*
chemical_list: D004872:Ergocalciferols; D007457:Iodine Radioisotopes; D014807:Vitamin D; D002762:Cholecalciferol; C104450:25-hydroxyvitamin D