A comparison of changes in body nitrogen measured by in vivo neutron activation analysis and urinary nitrogen balance

Abstract

Changes in total body nitrogen, and hence protein, can be estimated either from nitrogen balance, i.e. the difference between measured intake and output of nitrogen, or from changes in total body nitrogen measured serially by in vivo neutron activation analysis (IVNAA). The accuracy and precision of these two methods have been compared by using both to measure changes in the nitrogen content of 20 patients (10 men and 10 women) fed exclusively by the intravenous route. Nitrogen intake was calculated from the known intake and composition of the nutrient solution and nitrogen output was determined by measuring nitrogen in urine collected over periods of 24 h. IVNAA showed a statistically insignificant mean loss of 23 g of nitrogen, whereas the balance method gave a statistically significant mean gain of 77.4 g of nitrogen; the range of individual changes measured by neutron activation (−260− + 208 g) was four times the range found by the balance method (33–150 g). Estimated losses of nitrogen from the skin, and in faeces and blood, together with losses during intravenous feeding, might account for as much as 38 g of the difference between the mean changes observed by the two methods. A further 31 g might have been lost as a result of incomplete collection of urine, but the remaining discrepancy of 31 g is unexplained. The balance technique measures change in nitrogen more precisely than IVNAA but is more prone to systematic errors which result in an erroneously positive balance.