Flame measurement of the serum level of potassium one of the most accurate of laboratory tests. However accurately performed, the serum potassium determination has certain limitations. Thus, one must bear in mind that the serum potassium level measures the concentration of potassium in the extracellular fluid, not the total body potassium or the potassium of the cells. In an adult whose total body potassium is 3,000 mEq, only 50 to 70 mEq normally is present in the extracellular fluid. This extracellular potassium slowly exchanges with the cellular potassium through the permeable cell membrane when stimulated by changes in the pH of the extracellular fluid.
Potassium in Transit
Of several factors that determine the extracellular level of potassium, only one is the cellular store. Since the cells gain or lose potassium via the extracellular fluid—a sort of middleman fluid—the serum potassium level really represents potassium in transit. A low, normal, or
Snively WD, Westerman RL. Serum Potassium DeterminationA Useful Laboratory Tool. JAMA. 1966;197(7):579. doi:10.1001/jama.1966.03110070103026
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