Determination of 24,25-dihydroxyvitamin D 3 in Vitamin D External Quality Assessment Scheme samples using a reference measurement procedure

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 .