A potassium test is used to measure the amount of potassium in your blood. It is used to detect concentrations that are too high (hyperkalemia) or too low (hypokalemia). High serum potassium (hyperkalemia) occurs at potassium levels above 5.0 mM; it is considered severe over 8.0 mM. Hyperkalemia is relatively uncommon, but sometimes occurs in patients with kidney failure who take potassium supplements. Hyperkalemia can result in abnormal beating of the heart (cardiac arrhythmias). Low serum potassium (hypokalemia) occurs when serum potassium falls below 3.0 mM. It can result from low dietary potassium, as during starvation or in patients with anorexia nervosa; from excessive losses via the kidneys, as caused by diuretic drugs; or by diseases of the adrenal or pituitary glands. Mild hypokalemia causes muscle weakness, while severe hypokalemia can cause paralysis, the inability to breathe, and cardiac arrhythmias.The most common cause of hyperkalemia is kidney disease. Hypokalemia can occur if someone has diarrhea and vomiting or if is sweating excessively. It is a part of electrolyte panel. This test is usually ordered together with other electrolytes to get the better picture of underlying disorder.