The Vitamin D Metabolite Ratio (VMR) is a Biomarker of Vitamin D Status That is Not Affected by Acute Changes in Vitamin D Binding Protein

Background 25-hydroxyvitamin D[25(OH)D] may be a poor marker of vitamin D status due to variability in levels of vitamin D binding protein (VDBP). The vitamin D metabolite ratio (VMR) is the ratio of 24,25-dihydroxyvitamin D[24,25(OH)(2)D-3] to 25(OH)D-3 and has been postulated to reflect vitamin D sufficiency independent of variability in VDBP. Therapeutic plasma exchange (TPE) is a procedure that removes plasma, including VDBP, and may lower bound vitamin D metabolite concentrations. Effects of TPE on the VMR are unknown. Methods We measured 25(OH)D, free 25(OH)D, 1,25-dihydroxyvitamin D[1,25(OH)(2)D], 24,25(OH)(2)D-3, and VDBP in persons undergoing TPE, before and after treatment. We used paired t-tests to assess changes in these biomarkers during a TPE procedure. Results Study participants (n = 45) had a mean age of 55 +/- 16 years; 67% were female; and 76% were white. Compared to pretreatment concentrations, TPE caused a significant decrease in total VDBP by 65% (95%CI 60,70%), as well as all the vitamin D metabolites-25(OH)D by 66% (60%,74%), free 25(OH)D by 31% (24%,39%), 24,25(OH)(2)D-3 by 66% (55%,78%) and 1,25(OH)(2)D by 68% (60%,76%). In contrast, there was no significant change in the VMR before and after a single TPE treatment, with an observed mean 7% (-3%, 17%) change in VMR. Conclusions Changes in VDBP concentration across TPE parallel changes in 25(OH)D, 1,25(OH)(2)D, and 24,25(OH)(2)D-3, suggesting that concentrations of these metabolites reflect underlying VDBP concentrations. The VMR is stable across a TPE session despite a 65% reduction in VDBP. These findings suggest that the VMR is a marker of vitamin D status independent of VDBP levels.