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Amino acid (AA) loss is not equivalent on continuous venovenous hemofiltration (CVVH) compared with continuous venovenous hemodiafiltration (CVVHD). Amino acid supplementation may be necessary to adjust for a greater clearance on CVVH to maintain nitrogen balance similar to that of CVVHD.To compare AA losses and nitrogen balance between CVVH and CVVHD in children with acute renal failure.Pediatric patients in the pediatric intensive care unit of a tertiary referral center.Prospective randomized crossover study in consecutive children who required hemofiltration.A total of 12 plasma clearance studies for AA and urea, consisting of 24-hr collections of ultrafiltrate and urine for nitrogen balance, was performed on six patients during CVVH and CVVHD. Patients received total parenteral nutrition (TPN) with caloric intake 20% to 30% above their resting energy expenditure measured by indirect calorimetry and 1.5 g/kg/day protein of TPN. Study conditions were comprised of 2 L/hr/1.73 m2 of dialysate or prefiltered replacement fluid and hemofilter flow rates of 4 mL/kg/min were maintained for all patients.Amino acid clearances were greater on CVVH than CVVHD, except for glutamic acid, where clearance was 6.73 ± 2.31 (SEM) mL/min/1.73 m2 on CVVH and 7.59 ± 2.79 mL/min/1.73 m2 for CVVHD (NS). The clearance difference between the two modalities was 30%. Urea clearance was equivalent (30.1 ± 1.74 mL/min/1.73 m2 and 29.0 ± .97 mL/min/1.73 m2) for CVVH and CVVHD, respectively. Amino acid loss on CVVH and CVVHD was similar (12.50 ± 1.29 g/day/1.73 m2 vs. 11.61 ± 1.86 g/day/1.73 m2, respectively), representing 12% and 11%, respectively, of the daily protein intake. The catabolic state, as measured by urea nitrogen appearance, was high for all patients during the 48-hr study period with a mean of 291 mg/kg/day during CVVH, and 245 mg/kg/day for CVVHD. Nitrogen balance varied from a negative 12.95 g/day/1.73 m2 to a positive 4.93 g/day/1.73 m2 on CVVH and a negative 7.69 g/day/1.73 m2 to a positive 5.50 g/day/1.73 m2 on CVVHD.Clearance of AA is greater on CVVH than on CVVHD, but no significant difference in AA loss was present between the two therapies. Nitrogen balance often is not met on either therapy when a standard 1.5 g/kg/day protein and a resting energy expenditure of 120% to 130% of calories is delivered by TPN.