title: 25-hydroxyvitamin D assay variations and impact on clinical decision making.
abstract: BACKGROUND
  Laboratories are increasingly shifting to new automated 25-hydroxyvitamin D (25-OHD) assays, with subsequent variability in results.
  
  
  OBJECTIVE
  We describe the experience at our center with such a shift and illustrate its clinical implications.
  
  
  METHODS
  25-OHD levels were measured in 494 patients using Immunodiagnostic Systems RIA (IDS-RIA) and DiaSorin Liaison assays. Sources of variability between the assays were investigated in a subset of 83 samples, retested in the reference laboratory in the United States, and by reviewing the performance reports issued by the International Vitamin D External Quality Assessment Scheme, DEQAS. 25-OHD cut-points for target levels were used to compare the two assays.
  
  
  RESULTS
  25-OHD concentrations were significantly lower when measured with Liaison as compared to IDS-RIA: mean bias was -5 ng/ml, range was -38.1 to 18.7 ng/ml, P<0.001; the absolute bias was independent of 25-OHD value. Interassay variability was also detected in values obtained in the reference laboratory and in DEQAS reports. Using 20 ng/ml as the target 25-OHD level, 52% of patients required treatment when tested by Liaison, as opposed to 36% by IDS-RIA (P<0.001). Using 30 ng/ml as the desirable level, the proportions were 79 and 64%, respectively (P<0.001). The two assays agreed in only 41-68% of subjects, proportions that depended on criteria used to define agreement.
  
  
  CONCLUSIONS
  A change in 25-OHD assays has a significant impact on results, patient classification, and treatment recommendations. Such variability cannot be ignored when deriving and applying vitamin D guidelines. It also renders universal assay standardization a pressing call.
authors.affiliation: Division of Endocrinology, Department of Internal Medicine, American University of Beirut-Medical Center, P.O. Box 11-0236, Riad El Solh, Beirut 1107 2020, Lebanon.
authors.forename: Maya
authors.initials: M
authors.lastname: Barake
authors.forename: Rose T
authors.initials: RT
authors.lastname: Daher
authors.forename: Ibrahim
authors.initials: I
authors.lastname: Salti
authors.forename: Najwa K
authors.initials: NK
authors.lastname: Cortas
authors.forename: Laila
authors.initials: L
authors.lastname: Al-Shaar
authors.forename: Robert H
authors.initials: RH
authors.lastname: Habib
authors.forename: Ghada El-Hajj
authors.initials: Gel-H
authors.lastname: Fuleihan
mesh_terms: D003657:Decision Making; D005260:Female; D006801:Humans; D007118:Immunoassay / Q000295:instrumentation / Q000379:methods / Q000592:standards*; D008297:Male; D012015:Reference Standards; D015203:Reproducibility of Results; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000032:analysis
chemical_list: D014807:Vitamin D; C104450:25-hydroxyvitamin D