Normal serum potassium (K+) level is:

• A. 3.5-5.5 mEq/L
• B. 2.5-4.0 mEq/L
• C. 4.5-6.0 mEq/L
• D. 3.0-4.5 mEq/L

Answer: (A)

Potassium is the main intracellular cation and is essential for upholding the resting membrane potential that underlies nerve, muscle, and especially cardiac electrical activity. The normal serum potassium level sits in a relatively narrow window, about 3.5 to 5.5 mEq/L, and staying within this range is crucial because even small shifts can have serious consequences for heart rhythm and muscle strength. When potassium falls below this range, cells become less excitable in a way that can cause muscle weakness and arrhythmias; when potassium rises above, excitability increases and may produce dangerous conduction abnormalities, such as peaked T waves, widened QRS, and potentially life-threatening arrhythmias.

Several factors influence serum potassium distribution between the intracellular and extracellular compartments, including acid-base status, insulin levels, and catecholamines. Acidosis tends to raise serum potassium by moving it out of cells, while alkalosis lowers it by shifting it into cells. In neonatal and critically ill patients, potassium is closely monitored because IV fluids, transfusions, and medications can push levels outside the normal range. Therefore, the established reference range to know for safe practice is 3.5 to 5.5 mEq/L.