Potassium blood test: what high or low potassium means

What potassium measures

Almost all of your body's potassium sits inside your cells. Only a small amount is in the blood, and your kidneys keep it there in a narrow range by removing the extra in urine. Small changes in blood potassium can affect heart rhythm, which is why this test matters.

It is tested with sodium, chloride and CO2 in an electrolyte panel or a CMP.

Why a high potassium can be a false reading

Because cells hold so much potassium, anything that breaks them or squeezes potassium out of them in the tube raises the result without changing what is in your body. This is called pseudohyperkalemia. Common reasons:

• Hemolysis, red blood cells breaking open in the sample. This is the most common cause.
• A tourniquet left on a long time or repeated fist clenching during the draw.
• A very high platelet or white blood cell count, since those cells release potassium as the blood clots.

If your report says the sample was hemolyzed, or the high result surprises your clinician, the usual next step is a clean repeat draw. Keep your hand relaxed during it.

What a true high potassium (hyperkalemia) can mean

Hyperkalemia is a potassium above 5.5 mmol/L. Causes include:

• Kidney disease, the most common cause, because the kidneys can't remove the extra.
• Medicines that reduce how much potassium the kidneys clear, such as ACE inhibitors, ARBs, potassium-sparing diuretics, NSAID pain relievers and trimethoprim.
• Addison disease, poorly controlled type 1 diabetes, burns, injuries or surgery.
• Potassium supplements or salt substitutes, especially with kidney disease or the medicines above.

What low potassium (hypokalemia) can mean

• Diuretics (water pills), which flush potassium into urine.
• Fluid loss from vomiting, diarrhea, heavy sweating or laxative overuse.
• Other medicines, including insulin, corticosteroids and some antibiotics.
• Adrenal disorders such as primary aldosteronism and Cushing syndrome.
• Low magnesium, alcohol use disorder and eating disorders.
• Natural black licorice, which can lower potassium. MedlinePlus advises avoiding it for at least two weeks before the test.

Below 3.0 mmol/L may be considered severe. Symptoms can include muscle cramps and weakness, constipation, fatigue and palpitations.

What to do with an abnormal result

1. Repeat a surprising high result with a clean draw before changing anything.
2. Check kidney function. Read potassium with creatinine and eGFR.
3. Review medicines and supplements with your clinician, including blood pressure drugs, diuretics, NSAIDs, potassium pills and salt substitutes. Don't stop or start any on your own.
4. Ask whether magnesium should be checked if potassium is low, since low magnesium can keep potassium low.

Get emergency care if abnormal potassium comes with chest pain, a racing, pounding or irregular heartbeat, fainting, trouble breathing, or severe muscle weakness. A potassium above 6.5 mmol/L, or any level with heart rhythm changes, needs immediate treatment.