Interlaboratory Variation of Vitamin D Metabolite Measurements

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Abstract
Interlaboratory variation of the measurement of 25-hydroxy vitamin D, 24,25-dihydroxy vitamin D and 1,25-dihydroxy vitamin D by 6 laboratories in the Netherlands was studied. Three different [human] serum samples and 2 different standard solutions of each metabolite were assayed. Substantial interlaboratory variation was found for the measurement of serum samples. The mean interlaboratory CV [coefficient of variation] for the 25-hydroxy vitamin D, 24,25-dihydroxy vitamin D and 1,25-dihydroxy vitamin D assays in the 3 sera were 48%, 38% and 20%, respectively. The measurement of standard solutions of all metabolites showed relatively little variation (mean CV 8%). The small number of samples allowed no evaluation of intralaboratory variation. The much higher CV''s of the measurements of serum samples, when compared to standard solutions, may be attributed to differences in extraction and purification procedures which are probably responsible for the presence of varying amounts of interfering substances during the final quantification of metabolites.