Background and objectives: Protein-bound solutes are poorly cleared by peritoneal dialysis. We examined the hypothesis that plasma concentrations of bound solutes would therefore rise as residual renal function is lost.Design, setting, participants, & measurements: Clearances of urea indican and p-cresol sulfate were measured in peritoneal dialysis patients with and without residual function.Results: In patients with residual function, protein binding restricted the peritoneal indican and p-cresol sulfate clearances to 0.3 ؎ 0.1 ml/min, as compared to the peritoneal urea clearance of 5.5 ؎ 1.1 ml/min. The urinary indican and p-cresol sulfate clearances of 2.7 ؎ 2.5 and 1.3 ؎ 1.0 ml/min were closer to the urinary urea clearance of 3.9 ؎ 2.2 ml/min, reflecting the superior ability of native kidney function to clear bound solutes. Urinary clearance thus provided the majority of the total indican and p-cresol sulfate clearances of 3.0 ؎ 2.5 and 1.6 ؎ 1.0 ml/min in patients with residual function but the minority of total urea clearance of 9.4 ؎ 2.2 ml/min. Loss of residual function lowered the total clearances for indican and p-cresol sulfate to 0.5 ؎ 0.2 and 0.4 ؎ 0.2 ml/min, whereas the urea clearance fell only slightly. However there was only a modest increase in the plasma indican level and no increase in the plasma p-cresol sulfate level in patients with no residual function because reduction in the daily removal of these solutes accompanied the reduction in their total clearance rates.Conclusions: Reduction in the removal of indican and p-cresol sulfate kept plasma levels from rising markedly when residual function was lost.

Pruritus is a common debilitating symptom experienced by hemodialysis patients. Treatment is difficult because the cause of uremic pruritus is not known. This study addressed the hypothesis that pruritus is caused by solutes that accumulate in the plasma when the kidneys fail. We sought to identify solutes responsible for uremic pruritus using metabolomic analysis to compare the plasma of hemodialysis patients with severe pruritus versus mild/no pruritus. Pruritus severity in hemodialysis patients was assessed using a 100-mm visual analogue scale (VAS), with severe pruritus defined as >70 mm and mild/no pruritus defined as <10 mm. Twelve patients with severe pruritus (Itch) and 24 patients with mild/no pruritus (No Itch) were included. Pre-treatment plasma and plasma ultrafiltrate were analyzed using an established metabolomic platform (Metabolon, Inc.). To identify solutes associated with pruritus, we compared the average peak area of each solute in the Itch patients to that of the No Itch patients using the false discovery rate (q value) and principal component analysis. Dialysis vintage, Kt/Vurea, and serum levels of calcium, phosphorus, PTH, albumin, ferritin, and hemoglobin were similar in the Itch and No Itch patients. Metabolomic analysis identified 1,548 solutes of which 609 were classified as uremic. No difference in the plasma or plasma ultrafiltrate levels of any solute or group of solutes was found between the Itch and No Itch patients. Metabolomic analysis of hemodialysis patients did not reveal any solutes associated with pruritus. A limitation of metabolomic analysis is that the solute of interest may not be included in the metabolomic platform’s chemical library. A role for uremic solutes in pruritus remains to be established.