Clinical laboratories require accurate and precise 25-hydroxyvitamin D (25-OHD) immunoassays to allow comparison of patient results with published decision limits. Automated immunoassays, radioimmunoassay, HPLC and HPLC-Tandem Mass Spectrometry (LC-MS/MS) are the most commonly used methods in routine laboratories. However, even latest generation assays show significant variation in performance. Variable assay performance is caused by differences in assay design and can be assessed by challenging the assays with samples containing common interferences, such as heterophile antibodies, C3-epimer or 25-OH vitamin D2. In addition, analyses of samples with high and low analyte concentration are helpful in identifying the weaknesses of assays. Participants in this webinar will learn how to assess assay performance and how to use this information for selecting an appropriate test for their laboratory.