Potassium, a metallic inorganic ion with atomic weight of 39, is the most abundant cation in the body. The vast majority of potassium is in the intracellular compartment with a small amount in the extracellular space. Normal serum potassium is 3.5 to 5.5 mEq/L; however, plasma potassium is 0.5 mEq/L lower.

Serum potassium is measured by the use of a flame photometer or ion-selective electrode.