Standardizing vitamin D assays: the way forward

Abstract

For a number of years it has been widely assumed that measurement of serum 25-hydroxyvitamin D [25(OH)D] concentration is the best approach to assessing an individual's vitamin D status. However, it has also been recognized that there is substantial within-assay variation in 25(OH)D measurement and even greater between-assay variability. Such assay variation clearly confounds attempts to define what constitutes the diagnosis of hypovitaminosis D. Importantly, assay variability makes pooling of 25(OH)D results from different studies in systematic reviews for the specific purpose of determining dose-response and/or clinical cut points at best problematic. Therefore, to develop and implement evidence-based clinical guidelines, it is essential that 25(OH)D measurement be standardized in both clinical and research laboratories. In this Perspective we outline a way forward toward achieving this goal-the Vitamin D Standardization Program (VDSP).

Keywords: 25-HYDROXYVITAMIN D; STANDARDIZATION; VDSP; VITAMIN D; VITAMIN D STANDARDIZATION PROGRAM.

Fig. 1

VDSP Steps to Standardization. The primary steps to standardization are as follows: (1) develop a reference system; (2) establish metrological traceability; and (3) verify "end-user" test performance. (1) Reference system: the key components of a reference system are the development of reference measurement procedures, and the prepara-tion of suitable materials that can be used to establish traceability. These can include assigning target values to single donor serum samples prepared using guidelines to assure commutability; development of standard reference materials (trademark of NIST and certified reference materials; and the development of accuracy-based EQA/PT programs. (2) The materials can then be used to establish traceability between laboratories not using an RMP and those who are. (3) Verifying "End-User" test performance is then essential to develop consistency across assays. This step can include a certification process as in the CDC’s Standardiza-tion-Certification Program or in a somewhat less expensive and less rigorous way, EAQ/PT (ie, DEQAS and CAP EQA/PT surveys) can be used to assess "End-User" performance and consistency across surveys. Given the cost of participating in a program like the CDC's, which requires yearly certification participation in DEQAS and/or CAP is an essential step in the standardization effort. VDSP = Vitamin D Standardization Program; NIST = National Institute for Standards and Technology; EQA/PT = external quality assessment-performance testing; RMP = reference measure-ment procedure; CDC = Centers for Disease Control and Prevention; DEQAS = Vitamin D External Quality Assessment Scheme; CAP = College of American Pathologists.

Fig. 2

VDSP Reference Measurement System and Traceability Scheme. This figure is useful because it emphasizes the central role of immunoassay manufacturers in the effort to standardize the measurement of vitamin D. All assay manufacturers should participate in the CDC's Standardization- Certification Program. This is especially important for the manufacturers in-house reference method and for assay measurement systems as they are being developed. This figure is also useful because it emphasizes the central role of accuracy-based PT program; ie, ones using commutable materials. Individual serum donor samples, and especially SRMs orother certified materials can and—especially SRMs and CRMs—be used in clinical and research laboratories. VDSP = Vitamin D Standardization Program; CDC = Centers for Disease Control and Prevention; PT = performance testing; SRM = Standard Reference Materials; CRM = Certified Reference Materials.