Simultaneous assay of K⁴², Na²⁴, and Br⁸² in body composition measurements

¹ Medical Service and Research Laboratory, Boston Veterans Administration Hospital, and Department of Medicine, Tufts University School of Medicine, Boston, Massachusetts

Simultaneous measurements of Na²⁴ and K⁴² specific activities may be based on chemical, physical, or metabolic differences between these two radioisotopes. This paper describes a rapid and precise technique based on both physical and metabolic differences. Two counters, a gamma-ray and a beta-ray counter, are used. Two samples are collected from each patient at tracer equilibrium (40 hr after administration of K⁴², 50 µc; Na²⁴, 30 µc; Br⁸², 20 µc): urine, which has a high chemical K: Na ratio, and serum, which has a low chemical K: Na ratio. The urine is counted in the beta-ray counter, which favors K⁴². The serum is counted in the gammaray counter, which favors Na²⁴. To calculate the specific activities, equations are used which contain appropriate decay corrections, to permit calculation of the individual specific activities even though simultaneous counting of the samples in the two detectors is not required. The laboratory procedure is simple and straightforward, and the procedure gives results which are comparable in accuracy to methods which use only one isotope at a time. When counters of great sensitivity are employed, the procedure permits both small doses of radioisotope and long equilibration periods (40 hr). Total exchangeable chloride can be estimated at the same time by Br⁸² dilution. The calculation has been programed for a large digital computer.

Submitted on September 27, 1962