Endocrine Abstracts

Vitamin D is a large group of structurally similar substances (calciferols) with diverse biological functions. Although approximately 40 different vitamin D metabolites have been determined so far, only 1,25-dihydroxyvitamin D was considered to be biologically active and the determination of 25-hydroxyvitamin D (25-OH D) served a tool to assess vitamin D supply. Recently it was found that also C3-epimers of vitamin D have their biological effect, however these effects differ from non-epimeric forms. The exact source of the epimers is not known but their proportion was found higher in mothers and newborns. The percentage of C3-epimers of the total 25-OH D range from 0% to approx. 60% for infants and from 0% to 45% for adults, respectively. Such a high percentage of epimers could lead to misclassification of vitamin D supply, as routinely used immunoanalytical methods are unable to distinguish epimers from non-epimeric forms of vitamin D and are burdened with the high proportion of cross-reactivity and interferences. Vitamin D assessment is a challenge mainly due to the low concentrations of analytes in blood, a large number of structurally similar metabolites and photo-instability of vitamin D molecules. Chromatographic separation of epimers combined with the mass spectrometry detection is recognized as the optimal method for the determination of vitamin D, however, in laboratory practice prevail immunochemical methods. In our study, we compared various methods and parameters associated with vitamin D in 80 serum samples. Surprisingly, the values ​​of 25-OH D measured by chromatographic method and ELISA did not correlate together, but we found significant correlations between 25-OH D measured using chromatography and free 25-OH D. The choice of method is absolutely crucial when planning studies focused on vitamin D. Wide range of commercially available kits and at the same time, the unsatisfactory standardization of methods provides results that differ significantly from one another and can lead to problematic interpretation of the study outcomes. The work was supported by a grant of MH CZ – DRO (Institute of Endocrinology – EU, 00023761).