Urinary Urea Nitrogen: Too Insensitive for Calculating Nitrogen Balance Studies in Surgical Clinical Nutrition

Abstract

Urinary urea nitrogen (UUN) has been used as an estimate of total urinary nitrogen (TUN) when calculating nitrogen output for nitrogen balance (NB) studies. UUN is assumed to constitute 80 to 90% of the total nitrogen output; when estimating TUN from UUN, UUN values are multiplied by 1.25 to correct for non-urea nitrogen components. In order to evaluate the validity of estimating total urinary nitrogen output from measured UUN in a clinical setting, 491 UUN:TUN paired studies were performed on 24-hour urine collections in general surgical/trauma patients who had measured TUN outputs ranging from 0.04 to 54.0 g/d. Assessment of 315 NB studies was done to compare NB values of those calculated by using UUN as an estimate of TUN with those calculated from measured TUN. Patients in both studies were subdivided into four stress categories, using TUN/day as the index: less than 5 g, 5-10 g, 10-15 g, and greater than 15 g. On average, 80 to 90% of TUN is represented by the UUN. However, in our patient population the variability ranged from 12 to 112%. If these UUN values are used as estimates for TUN in calculating NB, variations of up to 12 g/d would result. Application of the correction factor of 1.25 is not consistent in correcting for nonurea nitrogen components in this clinical setting. The use of actual rather than estimated TUN may be a more accurate and appropriate method than UUN when calculating NB.

Metabolic effects of intensive insulin therapy in critically ill patients

“…The use of leucine R a in this study as a measure of proteolysis allows for a more accurate estimation of protein flux than urinary nitrogen balance (31). Our data show that rates of proteolysis in patients in study 1 were higher than in control subjects who received similar insulin doses, even though endogenous glucose production rate was suppressed, which would reduce the requirement for gluconeogenic precursors.…”

How much protein and energy are needed to equilibrate nitrogen and energy balances in ventilated critically ill children?

“…Out of nine studies, only two cross-sectional studies measured TUN [17]. In others, TUN was estimated from the urinary urea nitrogen content, which may be very variable in critically ill patients [7]. In addition, because REE was not measured by indirect calorimetry in all studies, it was not possible to examine the influence of energy balance on nitrogen balance.…”

“…According to the World Health Organization (WHO) [6], the major method currently available to determine protein needs is to measure nitrogen balance, which requires a precise quantification of both nitrogen intake and nitrogen losses. Total urinary nitrogen (TUN) must be measured [6] and not estimated from the urinary urea content, which is highly variable in critically ill patients [7]. Similarly, determining the energy balance demands the precise measurement of resting energy expenditure (REE) by indirect calorimetry [4].…”

Nutrition Assessment of the Intensive Care Unit Patient

“…23 The relationship between urinary urea and urinary nitrogen is affected both by malnutrition and by acute illness, making urea nitrogen measurements difficult to interpret, especially in the presence of sepsis. 43 …”