A simple method for estimating nitrogen balance in hospitalized patients: a review and supporting data for a previously proposed technique.

Total urinary nitrogen (TUN) and urinary urea nitrogen (UUN) in a 24-hour urine collection were measured under a variety of clinical and nutritional conditions in 81 patients for 564 study days. The difference between TUN and UUN averaged 1.8 +/- 0.9 g/day (range 0.0-5.8 g/day) and was independent of the absolute value for UUN. Since UUN was found to correlate linearly with TUN (R = 0.98) over a wide range of values, it is possible to estimate an approximate TUN excretion by adding a constant to the nitrogen excreted as urea. On the basis of the measured mean difference, we suggest using an estimate of 2 g of nitrogen per day. We feel this will not seriously under- or overestimate urinary nitrogen excretion in the calculation of nitrogen balance for the clinical management of nutritional support therapies. Since all hospitals can perform this routine test, it provides an effective, simple, and rapid method to follow accurately the protein catabolic response during disease and response to nutritional therapy. This method, without the supporting data presented here, has previously been recommended [Blackburn et al, 1977, JPEN 1:11-22] and has become a widely used technique for nutritional assessment.