What is the normal range for adult serum potassium?

Normal serum potassium is 3.5-5.0 mEq/L (or 3.5-5.0 mmol/L). Potassium is the major intracellular cation and is critical for normal cardiac, neuromuscular, and cellular function. Hypokalemia (below 3.5) causes muscle weakness, cramping, arrhythmias (U waves on ECG, flattened T waves, ST depression), constipation, polyuria. Common causes: diuretics (loop and thiazide), vomiting, diarrhea, inadequate intake, insulin therapy, alkalosis. Hyperkalemia (above 5.0) causes muscle weakness, paresthesias, life-threatening cardiac arrhythmias (peaked T waves, widened QRS, ventricular fibrillation, asystole). Common causes: renal failure, ACE inhibitors/ARBs, potassium-sparing diuretics, rhabdomyolysis, hemolysis, acidosis. NCLEX frequently tests potassium because of its narrow therapeutic range and severe consequences of imbalance.