Background: 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. Methods: 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. Results: 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 μmol/day, equivalent to ~7% of the total circulating level of VDBP in healthy controls. There was no correlation between 25(OH)D and VDBP. Conclusions: 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.

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Pediatric Nephrology
Erasmus MC: University Medical Center Rotterdam

Prytuła, A., Wells, D., McLean, T., Balona, F., Gullett, A., Knott, C., … Shroff, R. (2012). Urinary and dialysate losses of vitamin D-binding protein in children on chronic peritoneal dialysis. Pediatric Nephrology, 27(4), 643–649. doi:10.1007/s00467-011-2045-0