25-hydroxyvitamin D assay variations and impact on clinical decision making

J Clin Endocrinol Metab. 2012 Mar;97(3):835-43. doi: 10.1210/jc.2011-2584. Epub 2012 Jan 11.


Context: Laboratories are increasingly shifting to new automated 25-hydroxyvitamin D (25-OHD) assays, with subsequent variability in results.

Objective/setting: 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.

Conclusion: 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.

MeSH terms

  • Decision Making*
  • Female
  • Humans
  • Immunoassay / instrumentation
  • Immunoassay / methods
  • Immunoassay / standards*
  • Male
  • Reference Standards
  • Reproducibility of Results
  • Vitamin D / analogs & derivatives*
  • Vitamin D / analysis


  • Vitamin D
  • 25-hydroxyvitamin D