This webinar builds and expands on the recent labroots webinar by Dr. Gary Horowitz to emphasize how relatively in expensive assay standardization procedures can be incorporated into clinical and research laboratories.  Serum 25-hydroxyvitamin D [25(OH)D] is a very difficult metabolite to measure.   Assay variation continues to confound the clinical assessment of vitamin D deficiency. The Vitamin D Standardization Program (VDSP) was founded in 2010 by the NIH Office of Dietary Supplements in collaboration with organization and researchers around the world to promote the standardized laboratory measurement of 25(OH)D.  In this webinar, we will discuss the (1) meaning of assay standardization, (2) VDSP Reference Measurement System; (3) Cost-effective Steps to Standardize 25(OH)D Measurement; (4) VDSP Performance Guidelines – A Weakness; and (5) Emerging vitamin D standardization issues.