Measurement of extracellular space by total body neutron activation

Abstract

Although total body neutron activation analysis (TBNAA) has been used primarily to measure total body Ca and Ni composition, the present study emphasizes the potential application of total body Cl- (TBCl) data for determining the volumes of body water compartments. A good correlation was found between extracellular water (ECW) volume based on TBCl/plasma Cl- concentration and ECW based on the radiobromide dilution value. An excellent correlation was found between total body K (TBK) and intracellular water (ICW) in 115 normal men and women; ICW (liters) = 0.007 TBK (meq) - 0.62. ICW was calculated as the difference between total body water (TBW) and ECW, with TBW measured by isotope dilution and ECW determined by TBCl/plasma chloride. The TBK/ICW ranged from 145 to 150 meq/l. There was a good correlation between TBW as measured by isotope dilution and TBW as calculated from the sum of ECW and ICW. ECW was derived from the TBCl/plasma Cl-, and the ICW was predicted from the TBK values based on the regression equation above. The relationship between exchangeable Na (Nae) and extracellular water Na (ECWNa) is quite good. Nae was measured by the 24-h plasma 24Na dilution technique and the ECWNa was derived by multiplying the ECW by the plasma Na concentration. TBNAA in combination with whole-body counting (WBC) will yield useful data on TBW, ECW, ICW and ECWNa. Furthermore, TBNAA and WBC have the advantage over conventional isotope dilution in that these are direct noninvasive measurements, where equilibrium of all elements is assured.