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

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


We describe the experience at our center with such a shift and illustrate its clinical implications.


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.


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.


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.

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