Proposal of a new formula to calculate total urinary nitrogen from urea urinary nitrogen

Abstract

Urea urinary nitrogen (UUN) and total urinary nitrogen (TUN) were determined in 15 patients to study their relationship mainly in highly catabolic patients. The subjects were: 7 multiple trauma, 3 autologous bone-marrow transplant, 1 tetanus, 4 postcholecystectomy patients; 180 determination were made, 111 of which in highly catabolic phase (TUN > 15 g/day). TUN and UUN are well correlated (r = 0.9742; p < 0.001) in the range 3 to 52 g/ day of TUN. Urea represents the 83.2 ± 9.4% (M ± SD). A double-linear formula to calculate TUN from UUN is proposed (for UUN ⩽ 15 g/ day: TUN = UUN × 1.17 + 0.7; for UUN > 15 g/ day: TUN = UUN + 4). The least square procedure was used to compare the differences between measured and calculated TUN, and the results obtained with the proposed double-linear formula were compared with that obtained employing pre-existing formulas (Lee's and Mackenzie's). Considering all ranges of values studied (3–52 g of TUN) the results obtained were significantly different (p < 0.005); dividing the levels of TUN in ⩽ 15 g; 16–29 g and ⩾ 30 g, the results were always better, but the statistical significancy was not always reached.