Human extracellular water volume can be measured using the stable isotope (Na2SO4)-S-34

The volume of human extracellular water (ECW) may be estimated from the sul fate space (SS). Although it may better approximate ECW volume than the bro mide space, a common alternative, SS measurement is limited by the need to administer a radioactive substance, sodium [S-35]sulfate. In this paper, we demonstrate the measurement of the SS using the stable isotope, sodium [S- 34]sulfate. Eight healthy nonobese men ingested 0.50-0.78 mg (3.47-5.42 mu mol) (Na2SO4)-S-34/kg body weight and 30 mg NaBr/kg body weight. Sulfate co ncentrations and (SO4)-S-34 enrichments were measured by electrospray tande m mass spectrometry before and during the 5 h after tracer administration. SS was calculated by linear extrapolation of the natural logarithm of serum (SO4)-S-34 concentrations obtained at h 2, 3 and 4 compared with h 3, 4 an d 5. The SS obtained using values between h 3 and 5 (187 +/- 17 mL/kg) was similar to published determinations using intravenous or oral radiosulfate, and was 80% of the simultaneously measured corrected bromide space (234 +/ - 10 mL/kg, P = 0.01). Oral sodium [S-34]sulfate administration is a suitab le technique for measuring ECW and avoids radiation exposure.