Determination of 24,25-dihydroxyvitamin D₃ in Vitamin D External Quality Assessment Scheme samples using a reference...

Ninety archived human serum samples from the Vitamin D External Quality Assessment Scheme (DEQAS) were analyzed using a reference measurement procedure (RMP) based on isotope dilution liquid chromatography - tandem mass spectrometry (ID LC-MS/MS) for the determination of 24,25-dihydroxyvitamin D₃ [24,25(OH)₂D₃]. These 24,25(OH)₂D₃ results, in conjunction with concentration values assigned using RMPs for 25-hydroxyvitamin D₂ [25(OH)D₂] and 25-hydroxyvitamin D₃ [25(OH)D₃], provide a valuable resource for assessing the accuracy of measurements for 24,25(OH)₂D₃ and for investigating the relationship between 24,25(OH)₂D₃ and 25(OH)D₃. Results for 24,25(OH)₂D₃ using the RMP were compared to DEQAS consensus values demonstrating that the consensus values were not sufficient to assess the accuracy of measurements among different laboratories and methods. A multivariable regression analysis approach using historical DEQAS consensus values for various total 25(OH)D assays was used to assess the contribution of 24,25(OH)₂D₃ concentration on the assay response. The response of several ligand binding assays for total 25(OH)D was shown to be impacted by the presence of 24,25(OH)₂D₃.