16.8: Measurement of serum 25‑hydroxyvitamin D (18b.9)
- Page ID
- 117080
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)There are three main methods for measuring serum 25(OH)D: immunoassay, high performance liquid chromatography (HPLC), and liquid chromatography with tandem mass spectrometry (LC-MS/MS) However, there are two main steps that must be undertaken before selecting a method for assaying total 25(OH)D in serum. The first step is verification of “fit-for-purpose” . This step is only necessary when an immunoassay is the intended method and is necessary because some immunoassays are not appropriate for certain patient population groups (e.g., those with certain disease states, pregnant women, and vegetarians such as vegans) (Sempos and Binkley, 2020). In addition, the chosen immunoassay should have an appropriate measurement range for the intended study population (e.g., measure 25(OH)D levels in persons who are deficient). The “fit for purpose” step can be accomplished by testing the immunoassay against a Vitamin D Standardization Program (VDSP) standardized LC-MS/MS assay.
The second step required is standardization, a process whereby the intended assay is calibrated to meet the VDSP performance criteria (i.e., total coefficient of variation (CV) < 10% and a mean bias with the range of –5% to +5%). Some assays, even when based on a similar methodology, are less accurate and precise, making comparison of data across assays or laboratories difficult. The gold standard analytical method is LC-MS/MS and that should always be the method of choice for national surveys. However, even the LC-MS/MS method must meet the assay standardization criteria of the VDSP. Serum total 25(OH)D measurements can be 'prospectively' standardized or 'retrospectively' standardized, using methods developed by VDSP; see Sempos et al., 2017 and Durazo-Arivizu et al., 2017 for more details. A standardized laboratory measurement is defined as one that provides the 'true' serum total serum 25(OH)D concentrations. Several NIST Standard Reference Materials — SRM 972a, 2973, and 1949 — which provide target values for 25(OH)D2 and 25(OH)D3 are available to check on the accuracy and precision of the chosen assay. NIST SRM 1949 also provides target values for serum total 25(OH)D values for the first, second, and third trimester of pregnancy and these should be used in studies of pregnancy.