ACCURACY OF URINARY UREA NITROGEN FOR PREDICTING TOTAL URINARY NITROGEN IN THERMALLY INJURED PATIENTS

Estimations of total urinary nitrogen from measured urinary urea nitro gen are commonly used in calculating nitrogen balance. Recently publis hed studies suggest the urinary urea nitrogen/total urinary nitrogen r elationship is inconstant and total urinary nitrogen must be directly measured in burned patients. This study addresses the relationship of urinary urea nitrogen to total urinary nitrogen after thermal injury. Two hundred random 24-hour urine collections obtained from 45 thermall y injured patients (mean burn size 59 +/- 28%, mean age 40.5 +/- 17.2 years) between 1 and 354 days postburn were analyzed for total urinary nitrogen and urinary urea nitrogen. Regression analysis relating tota l urinary nitrogen to estimated total urinary nitrogen (urinary urea n itrogen x 1.25) revealed a linear relationship (r = .936, p <.001). Th e mean urinary urea nitrogen/total urinary nitrogen ratio was 0.77 +/- 0.10 and was not significantly correlated with percent burn, age, or postburn day. Mean nitrogen balance calculated from measured urinary u rea nitrogen in these patients was -5.7 g, and that calculated from me asured total urinary nitrogen was -6.3 g. This difference, although st atistically significant, is of little consequence for clinical use. Co ntrary to recent reports, we found the urinary urea nitrogen to be suf ficiently predictive of total urinary nitrogen for practical applicati on, and do not consider routine total urinary nitrogen measurements ne cessary for the nutritional care of thermally injured patients.