Evolution and impact of Standard Reference Materials (SRMs) for determining vitamin D metabolites

The National Institute of Standards and Technology (NIST), in collaboration with the National Institutes of Health, Office of Dietary Supplements (NIH ODS), introduced the first Standard Reference Material® (SRM) for determining vitamin D metabolites in 2009 motivated by significant concerns about the comparability and accuracy of different assays to assess vitamin D status. After 14 years, a suite of five serum matrix SRMs and three calibration solution SRMs are available. Values were also assigned for vitamin D metabolites in five additional SRMs intended primarily to support measurements of other clinical diagnostic markers. Both the SRMs and the certification approach have evolved from significant exogenous serum content to primarily endogenous content and from value assignment by combining the results of multiple analytical methods to the use of measurements exclusively from reference measurement procedures (RMPs). The impact of the availability of these SRMs can be assessed by both the distribution information (sales) and by reports in the scientific literature describing their use for method validation, quality control, and research. In this review, we describe the development of these SRMs, the evolution in design and value assignment, the expansion of information reported, and SRM use in validating analytical methods and providing quality assurance within the vitamin D measurement community.

Estimation of measurement uncertainty for the quantification of protein by ID-LC-MS/MS

The emergence of mass spectrometry (MS)-based methods to quantify proteins for clinical applications has led to the need for accurate and consistent measurements. To meet the clinical needs of MS-based protein results, it is important that the results are traceable to higher-order standards and methods and have defined uncertainty values. Therefore, we outline a comprehensive approach for the estimation of measurement uncertainty of a MS-based procedure for the quantification of a protein biomarker. Using a bottom-up approach, which is the model outlined in the "Guide to the Expression of Uncertainty of Measurement" (GUM), we evaluated the uncertainty components of a MS-based measurement procedure for a protein biomarker in a complex matrix. The cause-and-effect diagram of the procedure is used to identify each uncertainty component, and statistical equations are derived to determine the overall combined uncertainty. Evaluation of the uncertainty components not only enables the calculation of the measurement uncertainty but can also be used to determine if the procedure needs improvement. To demonstrate the use of the bottom-up approach, the overall combined uncertainty is estimated for the National Institute of Standards and Technology (NIST) candidate reference measurement procedure for albumin in human urine. The results of the uncertainty approach are applied to the determination of uncertainty for the certified value for albumin in candidate NIST Standard Reference Material® (SRM) 3666. This study provides a framework for measurement uncertainty estimation of a MS-based protein procedure by identifying the uncertainty components of the procedure to derive the overall combined uncertainty.

Vitamin D Standardization Program (VDSP) intralaboratory study for the assessment of 25-hydroxyvitamin D assay variability and bias

An intralaboratory study assessing assay variability and bias for determination of serum total 25-hydroxyvitamin D [25(OH)D] was conducted by the Vitamin D Standardization Program (VDSP). Thirteen assays for serum total 25(OH)D were evaluated in a single laboratory including 11 unique immunoassays and one liquid chromatography - tandem mass spectrometry (LC-MS/MS) assay. Fifty single-donor serum samples, including eight samples with high concentrations of 25(OH)D₂ (> 30 nmol/L), were assigned target values for 25(OH)D₂ and 25(OH)D₃ using reference measurement procedures (RMP). Using four replicate measurements for each sample, the mean total percent coefficient of variation (%CV) and mean % bias from the target values were determined for each assay using the 50 single-donor samples and a 42-sample subset, which excluded 8 high 25(OH)D₂ concentration samples, and compared with VDSP performance criteria of ≤ 10 % CV and ≤ ±5 % mean bias. All 12 assays achieved the performance criterion for % CV, and 9 of the 12 assays were within ≤ ±5 % mean bias. The Fujirebio Inc. assay exhibited the lowest %CV and highest percentage of individual measurements within ≤ ±5 % mean bias. Ten immunoassays exhibited changes in response due to the high 25(OH)D₂ samples with Abbott, Biomérieux, DiaSorin, DIAsource, and IDS-iSYS assays having the largest deviations. The Fujirebio Inc. and Beckman Coulter assays were only minimally affected by the presence of the high 25(OH)D₂ samples. Samples with high concentrations of 25(OH)D₂ provided a critical performance test for immunoassays indicating that some assays may not have equal response or recovery for 25(OH)D₂ and 25(OH)D₃.