Development of Standard Reference Material (SRM) 2973 Vitamin D Metabolites in Frozen Human Serum (High Level)

The National Institute of Standards and Technology (NIST), in collaboration with the National Institutes of Health Office of Dietary Supplements and the Vitamin D Standardization Program, has recently issued a new serum-matrix Standard Reference Material (SRM): 2973 Vitamin D Metabolites in Frozen Human Serum (High Level). SRM 2973 was designed to provide a serum material with a total 25-hydroxyvitamin D [25(OH)D] concentration near 100 nmol/L to complement the existing serum-based SRMs with values assigned for total 25(OH)D between 20 and 80 nmol/L. Values were assigned for 25-hydroxyvitamin D₂ [25(OH)D₂], 25-hydroxyvitamin D₃ [25(OH)D₃], 3-epi-25(OH)D₃ , and total 25(OH)D [the sum of 25(OH)D₂ + 25(OH)D₃] using the NIST isotope dilution LC with tandem MS (MS/MS) reference measurement procedure (RMP) and related methods. SRM 2973 has a certified value of 98.4 ± 2.1 nmol/L for 25(OH)D₃ and reference values of 1.59 ± 0.05 nmol/L for 25(OH)D₂ and 5.23 ± 0.20 nmol/L for 3-epi-25(OH)D₃ . In addition, a candidate RMP for 24R,25-dihydroxyvitamin D₃ [24R,25(OH)₂D₃] based on LC-MS/MS was used to assign values to SRM 2973 and the existing SRM 972a Vitamin D Metabolites in Frozen Human Serum. Reference values for 24R,25(OH)₂D₃ were assigned to SRM 2973 (7.51 ± 0.26 nmol/L) and the four levels of SRM 972a: Level 1 (6.38 ± 0.23 nmol/L), Level 2 (3.39 ± 0.12 nmol/L), Level 3 (3.88 ± 0.013 nmol/L), and Level 4 (6.32 ± 0.22 nmol/L). The development of SRM 2973 [with a higher concentration of 25(OH)D₃] and the addition of values for 24R,25(OH)₂D₃ assigned to both SRM 972a and SRM 2973 provide laboratories involved in vitamin D measurements with improved QA tools.