The Potassium (Serum) test is a common blood test that measures the amount of potassium (K) in the liquid portion of your blood (serum). Potassium is an essential electrolyte that helps your nerves and muscles work properly, especially the heart muscle. It also helps regulate fluid balance and supports normal cell function throughout the body.

Your result is usually reported in milliequivalents per liter (mEq/L) or millimoles per liter (mmol/L). A “normal” range can vary slightly by lab, so your provider will interpret your value using the reference range on your report and your clinical situation.

Low potassium (called hypokalemia) can contribute to muscle weakness or cramps and may trigger abnormal heart rhythms (arrhythmias), particularly in people with heart disease or those taking certain medications. High potassium (called hyperkalemia) can also cause dangerous arrhythmias and may be related to kidney problems or medications that raise potassium. Because potassium levels can change quickly and significantly affect cardiac rhythm, abnormal results often require prompt medical follow-up.

Serum potassium is often included in an electrolyte panel or basic metabolic panel (BMP) and may be ordered as a standalone test when a clinician needs to evaluate electrolyte balance or heart and muscle symptoms. It is commonly checked in routine care as well as urgent settings.

Your clinician may order a potassium blood test if you have symptoms that could be related to an electrolyte imbalance, such as:

• Muscle weakness, cramping, twitching, or fatigue
• Numbness or tingling
• Heart palpitations, irregular heartbeat, or chest-related symptoms
• Lightheadedness or fainting (especially with suspected arrhythmia)

It is also commonly done to help diagnose or monitor conditions and situations such as:

• Kidney disease or reduced kidney function (the kidneys are the main way the body controls potassium)
• Dehydration or significant fluid losses
• Vomiting, diarrhea, or heavy sweating that can lower potassium
• Uncontrolled diabetes, especially with very high blood sugar or diabetic ketoacidosis (DKA)
• Adrenal or hormone disorders that affect salt and water balance (e.g., aldosterone-related conditions)
• High blood pressure treatment and medication monitoring

Many medications can raise or lower potassium, so testing is often used for safety monitoring. Examples include diuretics (“water pills”), ACE inhibitors, ARBs, potassium-sparing diuretics (e.g., spironolactone), certain heart medications, and potassium supplements. It may also be checked before surgery, during hospitalization, or when an ECG/EKG suggests electrolyte-related changes.

• Chronic kidney disease (CKD) and acute kidney injury (AKI)
• Heart rhythm disorders (arrhythmias)
• Heart failure
• Dehydration
• Gastrointestinal fluid loss (prolonged vomiting or diarrhea)
• Diabetic ketoacidosis (DKA) and poorly controlled diabetes
• Hyperaldosteronism (including Conn syndrome)
• Addison disease (primary adrenal insufficiency)
• Metabolic acidosis or metabolic alkalosis
• Medication-related potassium imbalance (e.g., diuretics, ACE inhibitors/ARBs, spironolactone)

• Monitoring electrolyte balance as part of general wellness labs
• Supporting cardiovascular health by reducing risk from potassium-related arrhythmias
• Tracking kidney health and safe kidney disease management
• Medication safety monitoring (diuretics, blood pressure medicines, potassium supplements)
• Managing blood pressure treatment plans that may affect potassium (K)
• Monitoring hydration and recovery after illness with vomiting/diarrhea
• Diabetes management support when illness or high blood sugars may shift electrolytes