Urinary and dialysate losses of vitamin D-binding protein in children on chronic peritoneal dialysis

Vitamin D deficiency is widely prevalent in chronic kidney disease [CKD] patients. The aim of our study was to determine whether losses of vitamin D binding protein [VDBP] in urine and dialysate contribute to circulating 25-hydroxyvitamin D [25OHD] levels in chronic peritoneal dialysis [PD] patients. Dialysate, serum, and urine VDBP levels were measured in 16 children on PD and compared with serum and urine VDBP in ten CKD4-5 patients. Serum VDBP levels were correlated with total circulating 25OHD and peritoneal VDBP losses. The mean age of the study population was 9.4 +/- 3.8 years and the median time on dialysis 7.5 (1-18) months. In CKD4-5 patients, urinary VDBP losses were > 300-fold higher than seen in age-matched healthy children and correlated with urinary albumin loss (p = 0.0008). There was a significant correlation between serum VDBP and total dialysate and urine losses of VDBP (p = 0.03, r = -0.53). Dialysate VDBP losses correlate with dialysate albumin loss (p = 0.01). VDBP losses in the long daytime dwell were higher than in the overnight drain (p = 0.04). Serum VDBP levels were lower in children with a longer dialysis vintage (p = 0.0004, r = -0.77). In PD patients, the mean total loss of VDBP in dialysate and urine was 1.91 +/- 1.6 mu mol/day, equivalent to similar to 7% of the total circulating level of VDBP in healthy controls. There was no correlation between 25(OH)D and VDBP. Peritoneal VDBP losses mirror both dialysate and urinary albumin losses, and are associated with a longer dialysis vintage but do not contribute to vitamin D deficiency in children on PD.