Inaccuracy of nitrogen balance determinations in thermal injury with calculated total urinary nitrogen

Many burn centers use nitrogen balance studies to estimate the adequacy of nutritional support. Nitrogen loss includes the sum of urinary urea nitrogen, nonurea urinary nitrogen, and losses from skin, wound, and stool. Urinary urea nitrogen is often used to calculate total urinary nitrogen by multiplying the urinary urea nitrogen by a factor of 1.25 to account for nonurea urinary nitrogen. This formula is appropriate when applied to a nonstressed individual who has fasted overnight but is not appropriate for patients who have undergone surgery or experienced trauma. We have undertaken this study to assess the predictability of this formula in patients with thermal injuries. Twenty-seven patients with major thermal injuries had random 24-hour urine collections, which were analyzed for both urinary urea nitrogen and total urinary nitrogen. In these patients with burns we found that urinary urea nitrogen represented approximately 65% of the directly measured total urinary nitrogen rather than 80% as assumed by the formula. This increase in the nonurea nitrogen loss is greater than that found after surgery or trauma. Individual measurements may underestimate losses by 20% to 60%. Directly measured total urinary nitrogen should replace calculated total urinary nitrogen as the index of urine nitrogen losses for nitrogen balance studies in patients with burns.