Accuracy of two-hour urine urea nitrogen determinations in critically ill patients

The accuracy of two-hour versus 24-hour urine urea nitrogen (UUN) determinations in critically ill patients was compared. A 24-hour urine collection for UUN determinations was obtained each day for five days in 20 patients who had been receiving parenteral nutrition at a constant rate for at least 36 hours. For each patient, the UUN value obtained from a two-hour sample was projected using the actual 24-hour urine output to determine an estimated daily UUN excretion. Creatinine clearance determinations were performed to evaluate the effects of impaired renal function on the correlation of two-hour and 24-hour UUN excretion. A significant correlation (r = 0.889) was found between the two-hour and 24-hour UUN excretions. However, in three patients with creatinine clearances less than 30 ml/min and two patients with gram-negative sepsis, correlations between two-hour and 24-hour UUN excretion were poor. Total urine volumes in the 20 patients varied considerably but did not affect the correlation between UUN determinations. A two-hour UUN determination may be a valuable tool for monitoring the nutritional status of critically ill patients. The rate of intravenous nutrition must be kept constant, however, to minimize diurnal variations in nitrogen excretion. Conditions such a shock, sepsis, or acute renal failure may limit the use of a shorter urine collection period for urea nitrogen determination.