Dimension EXL Vitamin D Total Assay
Why Are Standardized Reference Materials Important?
Standardization of reference materials for in vitro diagnostic tests helps to harmonize results across test manufacturers and methods to improve the diagnosis, treatment, and prevention of disease. Reference standards serve as an anchor that can provide comparability across time and methods. Developing reference standards for assays is not a universal or easy procedure; success of any international reference material depends on that material being commutable across all manufacturers and methods.
Impact of the Lack of Standardization for Vitamin D
Without an internationally recognized, commutable vitamin D reference material, biases in slopes across methods and manufacturers can be observed. Scatter is apparent in assay comparisons between manufacturers as well as between laboratory-developed LC/MS/MS assays; this scatter may be due to differences in the way the different manufacturers’ systems measure vitamin D, varying vitamin D releasing processes, and heterophilic interferences.
Vitamin D standardization is necessary to create the anchor vitamin D values need, and it is important that laboratories and clinicians know how their vitamin D assay is standardized.
The Vitamin D Standardization Program (VDSP)
VDSP is an initiative of the NIH Office of Dietary Supplements (NIH ODS) and a collaboration with the National Institute of Standards and Technology (NIST), the CDC, and Ghent University.
- VDSP, under the coordination of Christopher Sempos, PhD, NIH ODS, aims to standardize 25(OH)vitamin D measurement across methods and manufacturers.
- The NIST Reference Measurement Procedure (RMP) is the primary reference method for the measurement of total 25(OH)vitamin D. This method separates out the 3-epi-25(OH)vitamin D3 from the 25(OH)vitamin D3, ensuring that the 25(OH)vitamin D3 is not overestimated.
- A second method by Prof. Linda Thienpont, Ghent University, is also available. It is an ID-LC/MS/MS method for vitamin D in human serum that is traceable to NIST Standard SRM2972.1
CDC Hormone Standardization Program (HoSt)
Ongoing manufacturer certification for vitamin D standardization will be part of the CDC HoSt program, conducted by the CDC’s Clinical Chemistry Branch, Division of Laboratory Services, under the supervision of Hubert W. Vesper, PhD, Director, Clinical Standardization Programs, and Chief, Protein Biomarker Biomarker and Lipid Reference Laboratory. Previous HoSt program assays include testosterone and estradiol.
The CDC Standardization Program for vitamin D is a two-phase process:
- First, assay manufacturers submit initial sample results for calibration to the standard.
- Manufacturers then receive quarterly blinded challenges to verify and monitor stability of calibration over time.
- The performance criteria are a mean bias of 5% and imprecision of 10%.
- After participants pass four consecutive surveys, they are awarded a certification for a year.2 Renewal is an annual process.
1 Scand J Clin Lab Invest. 2012;72(Suppl 243):41-49. 2
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