Abstract:
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,andby 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, 52percent of patients required treatment when tested by Liaison, as opposed to 36percent by IDS-RIA (P0.001). Using 30 ng-ml as the desirable level, the proportions were 79 and 64percent, respectively (P0.001). The two assays agreed in only 41-68percent 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 Dguidelines. It also renders universal assay standardization a pressing call. Copyright © 2012 by The Endocrine Society.