The precision of a modern method for body compartment measurements using multiple isotopes

Using readily available beta and gamma detectors, combined measurements of exchangeable sodium, exchangeable potassium, total body water and extracellular fluid volume using the isotopes 24Na, 43K, 3H and 35S were made in ten normal subjects. These measurements were repeated 1-6 weeks later so that an index of precision for each isotope study could be calculated. For such measurements the precision was 55 mmol for exchangeable sodium, 149 mmol for exchangeable potassium, 1.26 litres for total body water and 1.11 litres for extracellular fluid volume. The precision of our method is the best so far reported using readily available apparatus. These results may encourage further studies on different pathological conditions in man to be undertaken in peripheral laboratories.