Predicting total urinary nitrogen excretion from urinary urea nitrogen excretion in multiple-trauma patients receiving specialized nutritional support

Objective: We investigated the accuracy of methods to estimate total urinary nitrogen (TUN) excretion from urinary urea nitrogen (UUN) excretion for patients who have multiple trauma and receive specialized nutritional support. Methods: Fifty-five critically ill, adult patients who had multiple trauma and were receiving specialized nutritional support were evaluated. A 24-h urine collection for urea nitrogen and total nitrogen was performed 4.4 +/- 2.6 d after admission to the trauma intensive care unit. Patients with significant renal impairment, liver dysfunction, or obesity (>150% of ideal body weight) were excluded from study entry. Eight publications that examined the relation between TUN and UUN were evaluated for bias and precision in estimating TUN from UUN. Results: TUN was 20.8 +/- 10.8 g/d with an average difference of 3.8 +/- 2.8 g/d between TUN and UUN. Linear regression analysis comparing TUN with UUN indicated a significant correlative relation (TUN = 1.1 X UUN + 2; r = 0.958, P < 0.001). The difference between TUN and UUN varied based on UUN: for UUN lower than 10 g/d, TUN minus UUN was 1.5 +/- 1.0 g/d; for UUN 10 to 20 g/d, TUN minus UUN was 4.1 +/- 3.2 g/d; and for UUN higher than 20 g/d, TUN minus UUN was 5.3 +/- 1.9 g/d (P < 0.001). Six methods were biased toward underpredicting TUN, one method was unbiased, and one was biased toward overpredicting TUN. A practical method for estimating TUN from UUN was developed: TUN = UUN + 2 for those with UUN lower than 10 g/d and TUN = 1.1 X UUN + 2 for those with UUN of at least 10 g/d. Conclusions: Our method, the modified Velasco method, UUN/0.84, and UUN/0.85 provided reasonable estimates of TUN from UUN in critically ill, adult patients who had multiple trauma and were receiving specialized nutritional support; however, our method requires further validation. (C) 2005 Elsevier Inc. All rights reserved.