Comparison of Three Methods for the Estimation of Total Nitrogen Losses in Hospitalized Patients

Since the measurement of total nitrogen output (TNO) is not routinely determined in the clinical setting, its level is frequently estimated using formulas based on the urinary urea nitrogen excretion (UUN). We measured TNO in 124 surgical patients over 990 days (TNO, 19.22 ± 8.72 g N/ day; total urinary nitrogen (TUN) 18.17 ± 8.70 g N/day; UUN, 15.17 ± 7.70 g N/day; mean gastrointestinal nitrogen (MGIN) 0.68 ± 0.49 g N/day; integumental nitrogen (ITGN), 0.34 ± 0.08 g N/day) and compared the results with the daily estimations using three different formulas: formula A, UUN + 4; formula B, UUN x 1.20 + 1.05, where 1.20 is the reciprocal of the mean ratio UUN/TUN and 1.05 the mean extraurinary nitrogen losses; and formula C, UUN x 1.0986 + 2.55, derived from the regression analysis of UUN us TNO. TNO estimated by these formulas were 19.17 ± 7.70, 19.26 ± 9.24, and 19.22 ± 8.70 g N/day, respectively. The regression analyses of the estimated TNO from the three formulas versus the measured TNO indicated that formulas A, Band C were equally accurate in estimating TNO over the entire range of UUN. However, when only values of UUN ≥ 30 g N were considered, a modified formula A (UUN + 6) was the best predictor of TNO. Daily audits of the differences between the estimated and measured TNO showed comparable results for the three formulas. In 28.4 to 31.1 % of the observed days the differences were higher than ± 2 g N/day, an error which is not acceptable when estimating the protein requirements in many clinical conditions. Thus, the formulas for estimating TNO are only clinical approximations of the nitrogen losses. (Journal of Parenteral and Enteral Nutrition 14: 517–522, 1990)