Potassium test

Definition

This test measures the amount of potassium in the blood. Potassium (K+) helps nerves and muscles communicate. It also helps move nutrients into cells and waste products out of cells.

Potassium levels in the body are mainly controlled by the hormone aldosterone.

See also: Aldosterone test

Alternative Names

Hypokalemia test; K+

How the test is performed

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How to prepare for the test

The health care provider may tell you to stop taking any drugs that may affect the test.

Drugs that can increase potassium measurements include:

• Aminocaproic acid
• Antineoplastic drugs
• ACE inhibitors
• Certain diuretics
• Epinephrine
• Heparin
• Histamine
• Isoniazid
• Mannitol
• Succinylcholine

Drugs that can decrease potassium measurements include:

• Acetazolamide
• Aminosalicylic acid
• Amphotericin B
• Carbenicillin
• Cisplatin
• Certain diuretics
• Insulin
• Laxatives
• Penicillin G
• Phenothiazines
• Salicylates
• Sodium polystyrene sulfonate

The following factors can interfere with the test:

• Infusion of potassium-containing fluids
• Infusion of glucose or insulin

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 routinely done as part of an electrolyte level test.

Your doctor may order this test to diagnose or monitor kidney disease. The most common cause of high potassium levels is kidney disease.

Because potassium is important to heart function, your doctor may order this test if you have signs of high blood pressure or heart problems. Small changes in potassium levels can have a big effect on the activity of nerves and muscles, especially the heart. Low levels of potassium cause increased heart muscle activity, which can lead to an irregular heartbeat. High levels cause decreased heart muscle activity. Either situation can lead to a heart attack in some cases.

It may also be done if your doctor suspects metabolic acidosis (for example, caused by uncontrolled diabetes) or alkalosis (for example, caused by excess vomiting).

Occasionally, the potassium test may be done in persons who are having an attack of paralysis.

Normal Values

The normal range is 3.7 to 5.2 mEq/L.

Note: mEq/L = milliequivalent per liter

What abnormal results mean

High levels of potassium (hyperkalemia) may be due to:

• Addison's disease (rare)
• Blood transfusion
• Crushed tissue injury
• Hyperkalemic periodic paralysis
• Hypoaldosteronism (very rare)
• Kidney failure
• Metabolic or respiratory acidosis
• Red blood cell destruction

Low levels of potassium (hypokalemia) may be due to:

• Chronic diarrhea or use of laxatives
• Cushing syndrome (rare)
• Diuretics such as hydrochlorothiazide, furosemide, and indapamide
• Hyperaldosteronism
• Hypokalemic periodic paralysis
• Not enough potassium in the diet
• Renal artery stenosis
• Renal tubular acidosis (rare)
• Vomiting

Additional conditions under which the test may be performed:

• Acute adrenal crisis
• Acute bilateral obstructive uropathy
• Acute nephritic syndrome
• Bulimia
• Chronic kidney failure
• Cushing's disease
• Cushing syndrome caused by adrenal tumor
• Cushing syndrome - exogenous
• Cushing syndrome - ectopic
• Diabetic ketoacidosis
• Primary thrombocythemia
• Renal tubular acidosis - distal
• Rhabdomyolysis
• Thyrotoxic periodic paralysis

What the risks are

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Special considerations

If it is difficult to get the needle into the vein to take the blood sample, injury to the red blood cells may cause potassium to be released, causing a falsely high result.

References

Seifter JL. Potassium disorders. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 118.