Vitamin D and vitamin-D-binding protein kinetics in patients treated with continuous ambulatory peritoneal dialysis (CAPD)

Serum and dialysate levels of 25-hydroxycholecalciferol (25-OHD₃), 1,25 dihydroxycholecalciferol (1,25-(OH)₂D₃), and vitamin D-binding protein (DBP) were measured in 14 patients undergoing continuous ambulatory peritoneal dialysis (CAPD). Serum levels of 25-OHD₃ and DBP were within normal range (29.1 ± 22.9 nmol/L and 5.9 ± 1.1 μmol/L, respectively). Serum levels of 1,25-(OH)₂D₃ were subnormal in all (<16 pmol/L) but one. In 5 patients, dialysate concentrations of 25-OHD₃ were 2.3 ± 0.9 nmol/L, the rest had levels <1.0 nmol/L. Small quantities of 1,25-(OH)₂D₃ were found in the dialysate effluents. DBP could be detected in the dialysate in all patients (0.24 ± 0.06 μmol/L). Mass transfer (MT) of 25-OHD₃ and DBP were respectively -10.4 ± 8.3 nmol/24 h and -1.46 ± 0.46 μmol/24 h. Peritoneal clearances of 25-OHD₃ and DBP were low (0.40 ± 0.37 mL/min and 0.18 ± 0.06 mL/min, respectively. We conclude that CAPD leads to losses of 25-OHD₃ and DBP. However, the peritoneal loff of DBP is well compensated and does not result in serum deficiency. Serum 25-OHD₃ levels did not correlate with time on CAPD.