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.
When used for physiologic fluid analysis, shifts in retention times, even at constant flow rates, can cause integrators to function inadequately as tools for peak identification and thus result in concentration errors. A system capable of data collection, identification, quantification, and long-term storage that primarily uses retention times only for identification of elution sequence is described. Operator identification, aided by manipulation of x and y coordinates of the chromatograph, enhances positive identification of compounds for the experienced chromatographer.
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