Expert Access - Vitamin D Assays and What They Really Measure

Eugenio Zabaleta

Veronica I. Luzzi, PhD, DABCC

February 17, 2011

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Vitamin D Assays and What They Really Measure

Testing for vitamin D has become a very popular request in the last few years. Along with this popularity, a concern for the accuracy of available tests has also increased. There is concern because what we commonly call vitamin D is actually a class of structurally similar molecules, the two major physiologically relevant forms of which are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). A metabolite of vitamin D3, 25-hydroxy vitamin D (calcidiol), is the analyte recommended to assess vitamin D deficiency or toxicity.

However, several studies have shown that vitamin D assays, in particular competitive binding assays, may be biased to measuring one or the other of these two entities. These findings become relevant when trying to monitor vitamin D2 therapy using an assay that is biased to vitamin D3. In recent years, certain commercially available assays were under scrutiny because they lacked the accuracy to measure vitamin D2. During this presentation, I will compare and contrast different methodologies and discuss the consequences of using an inaccurate assay. Finally, I will review the external quality programs available to monitor accuracy and assay bias.



Dr. Luzzi is Associate Director of Chemistry in the Department of Pathology & Laboratory Medicine at Henry Ford Hospital, and Adjunct Professor of Pathology at Wayne State University, in Detroit, Michigan. She received her PhD in Biology and Physiology and Biophysics from the University of California, Irvine. Her focus is laboratory quality, standardization and integration using Lean management tools.

Dr Luzzi has worked in different areas of clinical testing and has published her work in Clinical Chemistry, The Journal of Biological Chemistry, Analytical Chemistry, and Molecular Pathology. Currently, she is actively involved in the Michigan chapter of the American Association for Clinical Chemistry and is a Diplomate and board member of the American Board of Clinical Chemistry.

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