The National Institute of Standards and Technology (NIST) has developed Standard Reference Material (SRM) 972a Vitamin D Metabolites in Frozen Human Serum as a replacement for SRM 972, which is no longer available. SRM 972a was developed in collaboration with the National Institutes of Health's Office of Dietary Supplements. In contrast to the previous reference material, three of the four levels of SRM 972a are composed of unmodified human serum. This SRM has certified and reference values for the following 25-hydroxyvitamin D [25(OH)D] species: 25(OH)D2, 25(OH)D3, and 3-epi-25(OH)D3. The value assignment and certification process included three isotope-dilution mass spectrometry approaches, with measurements performed at NIST and at the Centers for Disease Control and Prevention (CDC). The value assignment methods employed have been modified from those utilized for the previous SRM, and all three approaches now incorporate chromatographic resolution of the stereoisomers, 25(OH)D3 and 3-epi-25(OH)D3.

To reduce the variability in estradiol (E2) testing and to assure better patient care, standardization of E2 measurements has been recommended. This study aims to assess the accuracy and variability of E2 measurements performed by 11 routine immunological methods and 6 mass spectrometry methods using single donor serum materials and to compare the results to a reference method. The contribution of calibration bias, specificity or matrix effects, and imprecision on the overall variability of individual assays was evaluated. This study showed substantial variability in serum E2 measurements in samples from men and pre- and post-menopausal women. The mean bias across all samples, for each participant, ranged between -2.4% and 235%, with 3 participants having a mean bias of over 100%. The data suggest that calibration bias is the major contributor to the overall variability for nine assays. The analytical performances of most assays measuring E2 concentrations do not meet current needs in research and patient care. Three out of 17 assays would meet performance criteria derived from biological variability of +12.5% bias at concentrations >20pg/mL, and a maximum allowable bias of +2.5pg/mL at concentrations <20pg/mL. The sensitivity differs highly between assays. Most assays are not able to measure E2 levels below 10pg/mL. Standardization, specifically calibration to a common standard by using panels of individual patient samples, can reduce the observed variability and improve the utility of E2 levels in clinical settings.