Indirect measurement of total exchangeable potassium

A technique is described for the indirect measurement of total exchangeable potassium (Ke) that involves neither the rapidly decaying radioisotopes of potassium nor a total body counter. Total exchangeable sodium (Nae) and total body water (TBW) are measured by isotope dilution. Ke is calculated from Ke = R (TBW) - Nae, where R is the ratio of the sodium plus potassium content divided by the water content, in a sample of whole blood. In 19 normal rats and four rats with uremia there was an insignificant difference between the indirect determination of Ke and its direct measurement by carcass analysis. A similar insignificant difference was recorded between the indirect determination of Ke and its direct measurement by potassium-42 dilution in 15 normal dogs, five dogs with uremia, 14 dogs in an hyposmolar state, and in 20 patients. In 25 normal volunteers, Ke was determined indirectly. The regression of Ke on TBW and the 95% confidence intervals of this population was similar to published data in which Ke was determined by potassium-42 dilution. The indirect measurement of Ke was, therefore, validated experimentally.