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