Nitrogen balance studies and Kt/V urea in children undergoing chronic peritoneal dialysis

Currently, urea kinetic modeling has been accepted for the routine evaluation of peritoneal dialysis (PD) through the calculation of Kt/V (normalized whole body urea clearance) and nPNA (normalized protein equivalent of total nitrogen appearance). In pediatric dialysis, the exact meaning of and target values for, those parameters is still under debate. We evaluated the mean values and correlations between Kt/V urea and parameters of nutrition. During a 12-month period, we prospectively performed 186 nitrogen balance studies in pediatric patients on chronic PD. We also assessed daily protein intake (DPI) by nutritional evaluation. Protein, albumin, urea, and creatinine were analyzed in dialysate and urine, collected once monthly. Dialysis adequacy was evaluated using monthly measurements of Kt/V urea and creatinine clearance (CCr) in urine and dialysate. All statistical comparisons were performed using the paired t-test. Two-way analysis of variance for repeated measures was used to calculate correlations. A p value less than 0.05 was considered significant. We studied 20 patients (15 boys, 5 girls) of mean age 5.1 +/- 4.7 years (range: 3 months - 14.8 years). Mean DPI in these patients was 3.52 +/- 1.1 g/kg/day. Weekly total Kt/V urea was 3.41 +/- 1.35, and residual Kt/V urea was 1.69 +/- 1.46. Weekly total and residual CCr were 72.4 +/- 70 L and 45 +/- 44 L respectively. Mean protein catabolic rate (PCR) was 0.84 +/- 0.33 g/ kg/day, showing a net nitrogen balance (NB = DPI - PCR) of +1.37 +/- 0.4 g/kg/day. The mean nPNA was 1.38 +/- 0.40 g/kg/day, with positive correlations with DPI, PCR, and total Kt/V (p < 0.001). Total Kt/V showed significant positive correlations with PCR (p < 0.001) and nPNA (p < 0.001), but not with NB (p = 0.23) and DPI (p = 0.21). A negative correlation was found between all urea kinetic parameters and plasma bicarbonate (p < 0.001). The values of Kt/V urea and nPNA in our patients were higher than those recommended in the adult literature. The positive correlations seen between Kt/V urea and nPCR and nPNA could not be demonstrated between Kt/V and DPI or NB, suggesting that the correlations could be the result of a mathematical association. The negative correlations between plasma bicarbonate and urea kinetic variables suggest a negative impact of acidosis on nutrition status in our patients.