Typically, when serum potassium exceeds 5.5 mmol/L, it is referred to as hyperkalemia. However, an increase in serum potassium does not necessarily reflect an overall increase in body potassium; serum potassium can also rise when there is a deficiency of total body potassium. Therefore, in clinical practice, serum potassium is evaluated in conjunction with an electrocardiogram and medical history to determine if a patient has hyperkalemia. Hyperkalemia is an important emergency in internal medicine and can often lead to sudden cardiac arrest. It should be identified and prevented early.