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Substantial variability in 25(OH)D measurement continues to exist between assay methodologies.
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The Vitamin D Standardization Program (VDSP) has developed approaches allowing standardization of current and prior 25(OH)D data.
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It is necessary to use standardized 25(OH)D results in meta-analyses of outcomes potentially related to vitamin D status.
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Based on data from highly sun-exposed individuals, and including consideration of assay variability, it is our opinion that clinicians are well-advised to
Endocrinology and Metabolism Clinics of North America
Toward Clarity in Clinical Vitamin D Status Assessment: 25(OH)D Assay Standardization
Section snippets
Key points
Measurement of 25-hydroxyvitamin D
Serum 25(OH)D is notoriously difficult to measure; this is mainly caused by the hydrophobic nature of the molecule, its existence in three structurally different forms (25(OH)D3, 25(OH)D2, and 3 epi-25(OH)D), and its tight binding to vitamin D binding protein (DBP). Additionally, the presence of other substances in serum, particularly lipids, can compromise the ability of the binding agent, usually an antibody, to behave identically with the 25(OH)D in samples and standards. These so-called
What is assay standardization?
It is important to understand the meaning of assay standardization; for 25(OH)D a standardized laboratory measurement is one that is “comparable across measurement system, location and time.”21, 22, 23 Every laboratory regardless of the assay used would obtain the same 25(OH)D result (within predetermined statistical limits) on a given sample as would be obtained using one of the internationally recognized gold standard RMPs.24, 25, 26, 27 It is important to recognize that standardization is
Vitamin D Standardization Program
The VDSP was organized in 2010 by the National Institutes of Health Office of Dietary Supplements with the goal of promoting standardized total 25(OH)D measurement and thereby improving health around the world.12 As part of progress to meet this goal, the VDSP developed methods not only to standardize currently available 25(OH)D assays but also to retrospectively calibrate 25(OH)D results from prior studies that have stored serum to current reference measurement procedures. Such retrospective
Traceability
Ensuring that a 25(OH)D assay is traceable to the RMP is initially the responsibility of assay manufacturers and then subsequently of laboratorians directing their clinical laboratories.21 For health care providers, knowledge that their clinical assay is traceable to the RMP is important, but clinicians are also advised to appreciate that traceability includes guidance for assay bias and variability (% coefficient of variation [CV]). In this regard, the VDSP has adopted 25(OH)D assay
Effect of using standardized 25-hydroxyvitamin D assays
It is self-evident to clinicians that use of assays yielding different results on the same serum specimen could confound clinical care, particularly in the case of vitamin D status where fixed cutpoint values are being recommended to define inadequacy, such as 20 ng/mL or 30 ng/mL. As an example, consider the situation noted in Fig. 1 in which serum aliquots from two individuals were sent to several clinical laboratories. It is apparent that applying a rigid cutpoint to diagnose “low” vitamin D
Are all 25-hydroxyvitamin D assay methodologies the same?
The VDSP does not require any single analytical approach for 25(OH)D measurement. Indeed, the VDSP methods can be used in the standardization of any currently available 25(OH)D assay. However, it is important that clinicians appreciate differences in assay methodology, because despite following VDSP guidance, there are inherent differences. As a generalization, immunoassays have greater variability than chromatographic ones. This variability becomes clinically important despite following the
Performance testing/quality assessment of 25-hydroxyvitamin D assays
Participation in proficiency testing or external quality assessment schemes has made an important contribution to improving analytical performance in clinical laboratories. The DEQAS is the world’s largest specialist external quality assessment for vitamin D metabolites and, since 1989, the quarterly distribution of a large number of samples has facilitated the monitoring of trends in methodology and the performance of individual assays.8 DEQAS currently has approximately 1000 participating
25-Hydroxyvitamin D reference ranges
Clinical laboratories provide reference ranges to help clinicians interpret results. Traditionally, reference (“normal”) ranges are determined either by using the assay manufacturer’s normal range or from a sample of the local population. When the data are normally distributed, the normal range is defined as two standard deviations above and below the mean and encompasses approximately 95% of normal individuals. Unfortunately, because vitamin D deficiency is so common, such an approach would
What is a clinician to do?
Meta-analyses using unstandardized 25(OH)D results are unlikely to allow consensus to be reached. Indeed, it has been suggested that no further meta-analyses using unstandardized 25(OH)D data be performed.10 Moreover, many clinical trials of vitamin D supplementation performed in the past have flaws, most notably not requiring study subjects to be low in vitamin D before providing additional supplementation. Heaney41 recently published guidance for design and analysis of studies of nutrients,
Summary
Use of standardized 25(OH)D data is essential in ongoing and future research studies of vitamin D supplementation. Retrospective standardization of 25(OH)D data from key prior research studies is essential if those studies are to be used in systematic reviews attempting to define low vitamin D status. Without such standardization, efforts to develop guidelines regarding vitamin status are unlikely to reach consensus. Until prospective and retrospective standardized 25(OH)D data are available
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Cited by (49)
Determination of 24,25-dihydroxyvitamin D<inf>3</inf> in Vitamin D External Quality Assessment Scheme samples using a reference measurement procedure
2023, Journal of Steroid Biochemistry and Molecular BiologyWhat do you need to know about mass spectrometry? A brief guide for endocrinologists
2020, Annales d'EndocrinologieCitation Excerpt :Although the superior performance of LCMS for the determination of certain hormones is widely accepted, some authors pointed out some limitations in terms of analytical variability, especially in low concentrations, and standardisation [7–9]. These comparisons between several LCMS methods led to the implementation of standardisation programs [10,11]. A review of the literature by Tavita and Greaves shows that this lack of homogenisation of the results in LC-MSMS remains a critical issue leading to reference values, especially for steroids in paediatrics, which are still different from one laboratory to another [12].
Effects of enzyme-inducing antiseizure medication on vitamin D dosing in adult veterans with epilepsy
2020, Epilepsy ResearchCitation Excerpt :For these reasons, a serum concentration of greater than 30 ng/mL was defined as sufficient in order to be sensitive to individual patient differences. In addition, targeting a level of at least 30 ng/mL compensates for assay bias by assuring that all true serum levels will be above 20 ng/mL given the Office of Dietary Supplements Vitamin D Standardization Program (VDSP) recommends a 10 % coefficient of variation (CV) for clinical laboratories performing vitamin D assays (Binkley and Carter, 2017). Studies have shown a link between ASM use and increased bone turnover as evident by elevated osteocalcin and bone-specific alkaline phosphatase, two markers of bone formation (Pack, 2005; Sato et al., 2001; Verrotti et al., 2000).
Serum vitamin D concentrations at dry-off and close-up predict increased postpartum urine ketone concentrations in dairy cattle
2020, Journal of Dairy ScienceCitation Excerpt :The overall mean serum 25(OH)D is our sample was 90.2 ng/ml. This discrepancy could potentially be attributed to laboratory or assay variation, which is a common issue for serum 25(OH)D measurement in both humans and animals (Binkley and Carter, 2017). Circulating 25(OH)D concentrations are genotype-dependent, which makes it more difficult to compare concentrations between cows that may already have different baseline serum 25(OH)D concentrations (Casas et al., 2013).
Disclosure: The authors have nothing to disclose.