Urinary Nitrogen Constituents in the Postsurgical Preterm Neonate Receiving Parenteral Nutrition

Minimal information is available defining urinary nitrogen constituents in preterm neonates receiving parenteral nutrition (PN). The study objective was to evaluate 24-hour urine collections for total urinary nitrogen (TUN), urinary urea nitrogen (UUN), and the nitrogen content in creatinine, ammonia, free amino acids, protein, hippuric acid, and uric acid at baseline (days 1 to 2 of PN and days 1 to 3 after surgery) and 7 days later in eight preterm, postsurgical neonates. Calculation of undetermined nitrogen was also completed. Comparisons with historic, normal data were made for each urinary nitrogen constituent. At baseline, PN provided 59 ± 10 nonprotein kcal/kg·day ^-1 and 430 ± 54 mg/kg·day^-1. At day 7, PN provided 106 ± 23 nonprotein kcal/kg. day^-1 and 432 ± 30 mg/ kg·day^-1. TUN, UUN, and protein nitrogen decreased significantly from baseline at day 7 (p < .05). The percentages of TUN as amino acids, creatinine, and uric acid nitrogen were calculated. Percent amino acid nitrogen (6.0 ± 2.3% vs 8.4 ± 1.5%, p < .05), percent creatinine nitrogen (1.6 ± 0.5% vs 2.9 ± 0.8%, p < .001) and percent uric acid nitrogen (1.7 ± 0.9% vs 3.6 ± 2.1%, p < .05) increased significantly at day 7. The observed urinary free amino acid nitrogen fraction represented a higher percentage of TUN both at baseline and at day 7 when compared with term neonatal reference data, whereas creatinine nitrogen, uric acid nitrogen, and protein nitrogen represented a lower percentage of TUN. However, amino acid and creatinine nitrogen as a percentage of TUN were similar to levels in milk formula-fed preterm infants. The advancement of nonprotein calories and the likely decrease in physiologic stress at day 7 contributed to decreased TUN, UUN, and protein nitrogen excretion. Undetermined nitrogen remains a significant fraction of nitrogen losses in preterm neonates receiving PN. (Journal of Parenteral and Enteral Nutrition 17:68-72, 1993)