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 × 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 × 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.