In-vivo determinations of nitrogen in wasting disease

The 10.8-MeV capture gamma ray from nitrogen has been used to quantitate nitrogen in the human body. As a neutron source, four 5-Ci Pu-Be sources are used. These are contained in a heavily shielded box, with a collimator allowing a 20x20 cm2 area of the body to be exposed to a flux density of about 3x103n.cm-2.s-1. Capture gamma rays are detected in two 5 inx4 in NaI crystals which are themselves heavily shielded. In human studies, measurements are made of two areas of the body (chest and pelvis) for 10 min each. Typically a nitrogen net count of 1000-1500 is obtained in 10 min above a background of 600. Data is also obtained for the 2.2-MeV region of the gamma-ray spectra in order to quantitate hydrogen. By comparison of nitrogen and hydrogen counts and the use of body weight and the well-known ratio of hydrogen in the body, the mass of nitrogen in the body can be found. Potassium is measured in a whole-body counter. Correlations between nitrogen counts and body parameters of subjects have been investigated. As expected, N versus K gave the best correlation (r=0.88). The spread of N/K for normals is +-10% (2 SD). With patients with wasting disease the mean value of N/K is not significantly different from that in normals, but the range of values is much greater (SD of 20% and a range of a factor of two). These results show that despite the good correlation noted above, potassium cannot be used as a proper measure of protein. (author)