25-Hydroxyvitamin D Assay Variations and Impact on Clinical Decision Making

• Published in: The journal of clinical endocrinology and metabolism Vol. 97; no. 3; pp. 835 - 843
• Main Authors: Barake, Maya, Daher, Rose T., Salti, Ibrahim, Cortas, Najwa K., Al-Shaar, Laila, Habib, Robert H., Fuleihan, Ghada El-Hajj
• Format: Journal Article
• Language: English
• Published: United States Oxford University Press 01.03.2012; Copyright by The Endocrine Society
• Subjects: 25-Hydroxyvitamin D; Decision Making; Female; Humans; Immunoassay - instrumentation; Immunoassay - methods; Immunoassay - standards; Laboratories; Male; Patients; Quality control; Reference Standards; Reproducibility of Results; Vitamin D; Vitamin D - analogs & derivatives; Vitamin D - analysis
• ISSN: 0021-972X; 1945-7197; 1945-7197
• DOI: 10.1210/jc.2011-2584

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.