In the backwater of convective dialysis: decreased 25-hydroxyvitamin D levels following the switch to online hemodiafiltration

Background/aims: Vitamin D deficiency and elevated serum fibroblast growth factor-23 (FGF23) levels are hallmark features and surrogate markers of adverse clinical outcomes in patients with chronic kidney disease (CKD). Convection of molecules over the dialysis membrane during online hemodiafiltration (ol-HDF) increases the removal of larger waste molecules compared with traditional high-flux hemodialysis (HD). The primary aim of this study was to explore the long-term impact of ol-HDF on serum 25(OH) D and FGF23. Method: An observational, prospective, noncomparator study including 35 patients who were switched from HD to ol-HDF. Serum 25(OH) D and FGF23 were measured at baseline (i. e., time of switch to ol-HDF) and at 6, 12, and 24 months. Results: At follow-up time points, there was a significant reduction in serum 25(OH) D compared with baseline (p < 0.0001) whereas FGF23 was unaltered (p > 0.05). The decrease in 25(OH) D was more prominent in individuals with higher baseline 25(OH) D levels. Conclusion: OlHDF may lower systemic 25(OH) D levels by convective mechanisms although the clinical significance remains unknown. Further controlled studies are warranted to replicate these findings in larger patient cohorts.