Determination of 24,25-Dihydroxyvitamin D3 [24,25(OH)2D3] in Archived Vitamin D External Quality Assessment Scheme (DEQAS) Samples using a Reference Measurement Procedure with Comparison to DEQAS Participant Results and Assessment of Contribution of 24,25(OH)2D3 to DEQAS Assay Results

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 D 3 [24,25(OH) 2 D 3 ]. These 24,25(OH) 2 D 3 results, in conjunction with concentration values assigned using RMPs for 25-hydroxyvitamin D 2 [25(OH)D 2 ] and 25-hydroxyvitamin D 3 [25(OH)D 3 ], provide a valuable resource for assessing the accuracy of measurements for 24,25(OH) 2 D 3 and for investigating the relationship between 24,25(OH) 2 D 3 and 25(OH)D 3 . Results for 24,25(OH) 2 D 3 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) 2 D 3 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) 2 D 3 .