Middle molecule and small protein removal in children on peritoneal dialysis

Background. Dialysis efficiency has a great influence on the outcome of patients. Few data are available on the removal of solutes with molecular weights higher than urea and creatinine. The aim of our study was to assess the transport and the removal of substances with molecular weights up to 15 kD and to evaluate the contribution of residual renal function in peritoneal dialysis (PD) children. Methods. Seventeen patients of 12 ± 4 years undergoing automated PD were studied. Ten patients had 563 ± 355 mL/day of urine output, and 7 were anuric. During a standardized nightly intermittent PD (NIPD) session, a single-injection inulin clearance was performed. Urea, creatinine, inulin (measured by HPLC), cystatin C and β₂-microglobulin (β₂m) were measured in blood, urine and dialysate. Clearances (L/week/1.73 m²) and weekly solute removal index (SRI) were calculated for all the solutes; weekly Kt/V was calculated for urea. Results. In non-anuric versus anuric patients the total clearances were: urea 82.6 ± 18.3 versus 71.3 ± 26.4; creatinine 82.7 ± 28.6 versus 47.8 ± 18.8; inulin 42.8 ± 11.3 versus 32.8 ± 20.4; β₂m 14.2 ± 13.8 versus 9.2 ± 8.3; cystatin C 20.2 ± 9.4 versus 9.7 ± 4.8. In the patients with residual diuresis, the urea was removed mainly by PD (69.2%), while inulin, β₂m and cystatin C were removed by renal clearance (64.0%, 79.5% and 62.8%, respectively). Total, peritoneal and renal weekly Kt/V values in the subjects with residual renal function, were 2.86 ± 0.70, 1.99 ± 0.40 and 0.87 ± 0.43, respectively. Peritoneal weekly Kt/V in the anuric patients was 2.36 ± 0.85; total weekly Kt/V in the total group was 2.65 ± 0.78. Weekly SRIs in non-anuric versus anuric patients were: urea 2.56 ± 0.58 versus 2.09 ± 0.74; creatinine 2.66 ± 0.73 versus 1.46 ± 0.56; inulin 2.36 ± 0.92 versus 1.64 ± 1.60; β₂m 1.26 ± 1.10 versus 1.20 ± 1.90; cystatin C 1.72 ± 0.83 versus 1.58 ± 1.62. Conclusions. Solutes removed during PD tend to decrease following an increase in molecular weight of the substance. Since anuric patients are at higher risk of middle molecule and small protein accumulation, more attention should be paid to the removal of middle molecules. Further studies should be undertaken to evaluate whether removing them has a clinical impact and to determine their threshold levels.