A potassium test checks how much potassium is in the blood. Potassium is both an electrolyte and a mineral. It helps keep the water (the amount of fluid inside and outside the body's cells) and electrolyte balance of the body. Potassium is also important in how nerves and muscles work.

Potassium levels often change with sodium levels. When sodium levels go up, potassium levels go down, and when sodium levels go down, potassium levels go up. Potassium levels are also affected by a hormone called aldosterone, which is made by the adrenal glands.

Potassium levels can be affected by how the kidneys are working, the blood pH, the amount of potassium you eat, the hormone levels in your body, severe vomiting, and taking certain medicines, including potassium supplements. Certain cancer treatments that destroy cancer cells can also make potassium levels high.

Many foods are rich in potassium, including scallops, potatoes, figs, bananas, prune juice, orange juice, and squash. A balanced diet has enough potassium for the body's needs. But potassium is passed in the urine even if the level in the blood is low so the level of potassium in the body can get lower.

A potassium level that is too high or too low can be serious. Abnormal potassium levels may cause symptoms such as muscle cramps or weakness, nausea, diarrhea, frequent urination, dehydration, low blood pressure, confusion, irritability, paralysis, and changes in heart rhythm.

Other electrolytes, such as sodium, calcium, chloride, magnesium, and phosphate, may be checked in a blood sample at the same time as a blood test for potassium.

Why It Is Done

A blood test to check potassium is done to:

• Check levels in people being treated with medicines such as diuretics and for people having kidney dialysis.
• Check to see whether treatment for too low or too high potassium levels is working.
• Check people with high blood pressure who may have a problem with their kidneys or adrenal glands.
• Check the effects of extra nutrition (total parenteral nutrition [TPN]) on potassium levels.
• Check to see whether certain cancer treatments are causing too many cells to be destroyed (cell lysis). Cell lysis syndrome causes very high levels of some electrolytes, including potassium.

How To Prepare

You do not need to do anything before having this test.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

How It Is Done

The health professional drawing blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure to the site and then a bandage.