Serum potassium is a simple blood test that checks blood levels for the electrolyte potassium. This electrolyte is vital for the functioning of muscles and nerves. The test is commonly performed as part of an electrolyte panel or a routine checkup. Serum potassium levels give an assessment of heart health and kidney function. Abnormal potassium levels affect the heart, muscles and nerves. Your doctor may recommend testing potassium levels at regular intervals. Potassium tests may also be advised if you have diabetes or hypertension or are on certain medications that affect potassium levels.