Urinary ammonia plus urinary urea nitrogen as an estimate of total urinary nitrogen in patients receiving parenteral nutrition support.

Nitrogen balance has historically been estimated by using urinary urea nitrogen (UUN) multiplied by a factor of 1.25 to account for nonurea nitrogen present in the urine. Recently, the reliability of UUN as an estimate of nitrogen losses has been questioned and the use of total urinary nitrogen (TUN) has been proposed as a more accurate measure of urinary nitrogen losses. However, analysis of TUN losses is not readily available in many hospital laboratories. Because ammonia is the major fluctuating component of urinary nonurea nitrogen and equipment to measure urinary ammonia is available in most hospitals, this study was undertaken to determine whether urinary ammonia plus UUN provides a clinically useful approximation of TUN. Twenty-four-hour urine samples acidified with boric acid during collection from 20 patients (a total of 42 samples) receiving total parenteral nutrition were analyzed for UUN, ammonia, and TUN. The UUN values ranged from 4.9 to 42.4 g/24 h. The mean difference between TUN and UUN was 1.99 +/- 0.27 g/24 h. The mean difference between TUN and UUN plus ammonia was 0.78 +/- 0.27 g/24 h. Thus, UUN alone accounted for 90% and the combination of UUN plus urinary ammonia accounted for 96% of TUN. These data suggest that UUN plus ammonia does provide a greater level of reliability as an estimate of TUN than UUN alone.

Proficiency testing in a medical-needs context.

Proficiency testing by State and Federal agencies is an ongoing activity of clinical laboratories and an occasional source of anxiety; strict statistical evaluations of "snapshot" laboratory values are inappropriate and medical-needs criteria should be used. The quality of laboratory results largely depends on available technology. Fortunately, for most of the common clinical chemistry analytes there has been a steady reduction of imprecision during the past 20 years. Proficiency testing may have been the stimulus for this improved performance. Medical-needs criteria differ, depending on the testing goal. For proficiency testing, population screening criteria are appropriate, e.g., the College of American Pathologists fixed criteria for the common tests. Stricter criteria are needed for short-term (inpatient) and long-term (outpatient) monitoring of laboratory data. Explicit proficiency-testing limits are given here for nine of the common clinical chemistry tests for each of the three medical-needs criteria described above. The limits consider total error--i.e., bias from the believed correct value, and imprecision. Rather broad limits are acceptable for the commonly performed enzyme tests when used for screening purposes.