A simple calibration of a shadow shield counter for the measurement of total body potassium in critically ill patients.

A shadow shield counter has been built for the determination of total body potassium (TBK), based on the detection of the 1.46 MeV gamma rays emitted by the naturally occurring radioisotope (0.01%) 40K. Simple calibration procedures have been adopted which do not require administration of 42K to the subject. Self-absorption effects of gamma rays have been taken into account by monitoring the activity of 40K per gram of potassium as a function of weight. All activity measurements, either during calibration or of a subject, include background in the spectral band covering the full width at half maximum (FWHM) of the 40K photopeak. The overall precision for determining the TBK was in the range of 1.8-3.0%.