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The National Institute of Standards and Technology (NIST) has established a Vitamin D Metabolites Quality Assurance Program in collaboration with the National Institutes of Health (NIH) Office of Dietary Supplements (ODS). For this first pilot exercise (Winter 2010 Comparability Study), participants were asked to use the methodology of their choice to measure concentrations of 25-hydroxyvitamin D in control and study materials distributed by NIST. The study material consisted of triplicate samples of SRM 1950 Metabolites in Human Plasma. SRM 2972, which is comprised of separate ethanolic calibration solutions with known concentrations of 25(OH)D2 and 25(OH)D3, was provided as a control material. Participants provided their data to NIST, where it was compiled and evaluated for trueness relative to the NIST value, within-laboratory precision, and concordance within the participant community. A report of results was provided to all participants of the exercise, and laboratories were identified by code numbers known only to them. The results from this first pilot exercise are reported along with a summary of the analytical methods used.