Potassium Test (Serum Potassium Test / K⁺ Test) A potassium test measures the level of potassium in the blood. Potassium is an essential mineral and electrolyte that helps regulate nerve and muscle function, including the heart, as well as fluid balance in the body. Abnormal potassium levels can indicate conditions such as dehydration, kidney disease, heart problems, or issues caused by certain medications.