Potassium test
Alternative names:
hypokalemia test; K+
How the test is performed:
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
Infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.
Occasionally, in people experiencing periodic episodes of paralysis, the potassium level may be drawn during an attack of paralysis.
How to prepare for the test:
The health care provider may advise you to withhold drugs that may affect the test (see special considerations).
Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
How the test will feel:
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed:
This test is performed to evaluate blood levels of potassium.
Potassium (K+) is the major positive ion within cells and is particularly important for maintaining the electric charge on the cell membrane, which is necessary for neuromuscular communication and for transporting nutrients into cells and waste products out of the cell. The concentration of potassium inside cells is about 30 times that in the blood and other extracellular fluids.
Potassium levels are mainly controlled by the steroid hormone aldosterone (for more information see the aldosterone test), which increases excretion of potassium. Aldosterone is secreted from the adrenal gland in the presence of increasing levels of potassium. Metabolic acidosis (for example, caused by uncontrolled diabetes) or alkalosis (for example, caused by excess vomiting) can affect blood potassium because this ion shifts into or out of cells in exchange for hydrogen ions. For example, in acidosis some of the excess hydrogen ions will shift into cells in exchange for potassium ions being released from the cells.
Small changes in the potassium concentration outside cells can have substantial effects on the activity of nerves and muscles. This is particularly true of the heart muscle. Low levels of potassium cause increased activity (which can lead to an arrhythmia), whereas high levels cause decreased activity. Either situation can lead to cardiac arrest in some circumstances. In normal people, taking potassium supplements or potassium-containing drugs is of no consequences, because the kidneys efficiently dispose of excess potassium.
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