Measurement of circulating 1,25-dihydroxyvitamin D and Vitamin D-binding protein in chronic kidney diseases

Abstract

After providing a short reminder on the physiopathology of vitamin D, we will discuss the potential interest to analyze the different metabolites of vitamin D. Undoubtly, measuring 25(OH) vitamin D is one of the most relevant and frequent dosage in daily clinical practice. Indeed, this is the most interesting measurement to assess global vitamin D status. The number of 25(OH)D determinations has dramatically increased over the last 10 years. This fact has led most of the clinical laboratories to move from the DiaSorin RIA, the most widely used method before twenties, to automated immunoassays or liquid chromatographs coupled with two mass spectrometers in tandem (LC-MS/MS). Anyway, one has to remember that analytical 25(OH)D determination is far from an easy task. Indeed, several important problems have to be overcome to correctly assess this parameter. Among them, the very high lipophilic nature of the molecule and its strong association with vitamin D-binding protein (VDBP) and, to a lesser extent, albumin, necessitates a thorough separation step and, for the one-phase immunoassays, a good equilibrium between the analyte and the antibodies used in the kits. We will also discuss the performance of this measurement in very specific population like pregnant women and chronic kidney diseases patients treated by dialysis among others. In this chapter, we will also describe potential indications and methods available to measure other vitamin D metabolites like cholecalciferol, 1,25 and 24,25 vitamin D. Finally, we will critically focus on vitamin D binding protein (VDBP) and on the new concept of “free�? or “bioavailable�? vitamin D.