The 25-hydroxyvitamin D (25OHD) serum concentration is the marker of vitamin D status. It is measured by immunoanalysis, mostly on automated platforms, or by separative methods such as HPLC or LC/MS-MS. An important inter-method variability has been evidenced some years ago. This has however improved recently thanks to the availability of an international standard and of a reference method (RM), and to the implementation of the Vitamin D Standardization Program (VDSP). Some problems remain however, specially in conditions where the concentration of the vitamin D-binding protein is greatly modified, such as in late pregnancy, or when the altered composition of the serum induces a «matrix» effect, such as in chronic kidney disease (CKD). The RM is a LC/MS-MS method which uses a complicated sample preparation and an isotope dilution step. It is not designed for use in routine, and a frequent mistake is to consider that LC/MS-MS is (always) the RM. Indeed, while the results of the LC/MS-MS assays are usually better correlated with those of the RM than those of most immunoassays, a significant, and sometimes huge, bias exists between the results of many LC/MS-MS assays and the RM. The VDSP has validated a certification protocol to ascertain that a 25OHD assay is strictly comparable to the RM. An important question is «to which patients should we measure 25OHD ?». Clearly, the answer is to some extent a matter of opinion as evidenced by the very different recommendations published by many groups of experts. My personnal opinion is not to measure 25OHD in the general population but to focus on some patients such as those with (or at high risk of) osteoporosis, those with CKD, those with malabsorption, and in any exploration of calcium/phosphate metabolism when serum PTH is measured.
18 - 21 May 2019
European Society of Endocrinology