Ninety-five percent of body potassium is intracellular where it fulfills a role with respect to intracellular water, which is analogous in many ways to that of sodium in the extracellular water. For example, each is the principal determinant of the osmolality of its compartment, and the absolute quantities of each in the organism are clearly related to the volume of the intracellular and extracellular spaces. The ease with which the sodium concentration of the extracellular fluid (ECF) may be measured contrasts markedly with the difficulties of measuring directly the intracellular potassium concentration. This difficulty is central to many of the problems associated with the assessment of potassium stores. Nevertheless, in the assessment of potassium stores, it is not only necessary to know the amount of potassium in the body but to be able to express this figure in a physiological and meaningful way. This should ideally include a measurement of the concentration of potassium in intracellular water as well as the technically easier expression in terms of dry solids or body weight. Furthermore, since potassium is maintained within the cell by an active transport system which transports sodium outwards and potassium inwards, thus balancing "passive" leaks down the electrochemical gradients [1, 2], a proper appraisal of potassium physiology should include an assessment of this mechanism, both active transport and so-called passive permeability. In theory there ought to be at least two distinct types of potassium deficit leading to a low intracellular potassium concentration, one which follows negative balance from, for example, severe diarrhea, and a second which is consequent upon altered membrane permeability or disordered function of the sodium-potassium exchange mechanism and results from the consequent inability to retain potassium within the cell. A third mechanism for a reduction in total body potassium without a necessary reduction in intracellular potassium concentration is loss of lean body mass (LBM), as in muscle wasting.These different types of potassium deficiency and the different modes of expression of potassium measurements which are required to distinguish between them cannot be obtained from a single methodology. Thus, a clear formulation of the aspect of potassium stores under study and an understanding of which methodology will provide the answer are needed before data can be acquired and presented in an appropriate manner. Many of the problems and apparent contradictions in the literature on body potassium stores are the result of failures in formulation and interpretation rather than errors of measurement.For this reason this paper will detail the methods available for the assessment of potassium stores with particular reference to the limitations associated with each method. A tentative approach to the assessment of potassium stores will then be presented and its application in clinical settings briefly demonstrated.
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