1,25-Dihydroxyvitamin D Testing in Clinical Practice: A Literature Review on Diagnostic Challenges Arising From...

As our understanding of vitamin D and its metabolites has improved, different assays have been developed for measuring their serum concentrations. The predominant circulating metabolite, 25-hydroxyvitamin D (25(OH)D), has been extensively studied and is now widely used for assessing vitamin D status. Of current interest is the biologically active metabolite, 1,25-dihydroxyvitamin D (1,25(OH)₂D), which is involved in numerous physiological processes related to bone metabolism and various non-calcaemic functions. Quantification of 1,25(OH)₂D serves not only as a diagnostic tool for numerous diseases, but also as a way to ensure optimal levels in the circulation to maintain physiological functioning. Measuring 1,25(OH)₂D, however, proves challenging as its concentration in the blood is very low, and the molecule itself is highly lipophilic. A variety of analytical assays have been developed for measuring 1,25(OH)₂D, each differing in its analytical principles. These methods exhibit variations in their performances, showcasing both strengths and limitations in their underlying analytical principles; some of these assays continue to be used today, while others have become obsolete. Comparability of these methods to this day is challenging due to the absence of a standardisation scheme for 1,25(OH)₂D.