A small constant percentage of total body potassium (TBK) is radioactive (40K) and emits a 7-ray. With appropriate shielding from background, this 7-ray can be counted using scintillation detectors. As the ratio of 40K
to K is known and constant, K or ''total body potassium'' can be estimated accordingly. All of the body's potassium is within the FFM compartment and the proportion of the body FFM compartment TBK/FFM ratio is relatively stable in the same subject over time and between different subjects. However, with increasing age or when comparing young versus elderly, the TBK to FFM ratio decreases.
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