An evaluation of total 25-hydroxyvitamin D assay standardization: Where are we today?

•Only few 25(OH) D assays comply with requirements in the Vitamin D Standardization Program.•25(OH) assays vary greatly in the categorization of patients in vitamin D deficiency/sufficiency.•Standardization has been obtained in some 25(OH)D immunoassays. Serum total 25-hydroxyvitamin D is a measure of the total circulating 25-hydroxyvitamin D concentration and is the primary measurement for estimating vitamin D status. A number of automated immunoassays are commercially available, and in an attempt to standardize the assays the Vitamin D Standardization Program (VDSP) was established in 2010. Therefore, the aim of the current project is to evaluate the status of the standardization of routinely used 25-hydroxyvitamin D assays. 200 patient serum samples were measured in Spring 2017 on seven different assays for 25-hydroxyvitamin D. Samples were measured in duplicate for the evaluation of precision. A certified standard reference material (SRM972a) from The National Institute of Standardization and Technology (NIST) was measured to evaluate the accuracy of the assays. Finally, the agreement of the assays of clinically categorizing patients into vitamin D deficiency, inadequacy or adequacy was evaluated. All seven assays achieved precision below the VDSP requirement of CV