What Is a Potassium Blood Test?

What to expect when undergoing this test

In This Article

A blood potassium test is one of the most commonly ordered lab tests and may be done for a wide variety of medical conditions. As an electrolyte that is critical in muscle contraction and nerve conduction, both elevated and decreased levels can be very serious. We will look at the reasons why your doctor may recommend this test, the normal range in adults and children, potential causes of high (hyperkalemia) or low (hypokalemia) levels, and what further testing may be recommended depending upon your results.

A very high or very low blood potassium test can be a medical emergency.

Purpose of Test

Potassium is an extremely important electrolyte in the body, playing a crucial role in muscle contraction (both skeletal muscles and the heart muscle), the conduction of nerve impulses, and more. As such, it is ordered frequently in both the clinic and hospital setting.

The test may be ordered for a variety of reasons, including:

  • As part of a general chemistry or electrolyte panel during routine health exams
  • To evaluate and monitor many chronic conditions, including heart conditions, high blood pressure, lung conditions, kidney conditions, endocrine conditions, and more
  • To detect abnormalities in those who are experiencing vomiting, diarrhea, or dehydration
  • If you have symptoms of high or low potassium, such as muscle weakness, palpitations or an irregular heart rhythm on an EKG (it's important to note that abnormal potassium levels can be very serious even if symptoms are not present)
  • To monitor medications, especially those that can result in a high or low potassium level
  • To monitor acid-base balance in the body
  • When intravenous fluids are given
  • During cancer treatments that cause cell death (cell lysis or breakdown can result in the release of large amounts of potassium into the blood)

Significance of Blood Potassium

Evaluating blood potassium is very important in the management of many medical conditions, and can sometimes alert physicians to problems before potentially life-threatening complications develop. Potassium levels reflect how well the kidneys are working, the actions of hormones such as aldosterone in the body, the effect that medications may be having on the body, and how much potassium is taken into the body via the diet (although intake alone rarely causes abnormal levels when the kidneys are functioning well).

It's important to note, however, that while a blood potassium test is one of the more helpful tests, blood potassium levels do not necessarily represent the body stores or cellular levels of potassium. Only around 2% of potassium in the body is found in the blood. This concept must be kept in mind in conditions such as diabetic ketoacidosis, when body stores of potassium may remain critically low even if blood levels appear to return to normal.

Limitations

As with most medical tests, there are some limitations in interpreting potassium levels in the blood.

Results are less accurate (may be falsely elevated) in people who have a high white blood cell or platelet count.

There is also significant variability with potassium blood tests done on whole blood, according to a 2018 Mayo Clinic study. Both an inaccurate reading of high potassium (pseudohyperkalemia) and low potassium (pseudohypokalemia) occur frequently, and this finding needs to be considered when interpreting the test results.

Similar Tests

At the current time there is not an approved home potassium blood test, though this is being investigated.

Research is also in progress looking for non-invasive ways to detect elevated potassium levels via EKG readings. A 2019 study published in JAMA found that applying artificial intelligence to continuous or remote EKG monitoring may be helpful in the future for detecting even small changes in potassium level in people at high risk.

Complementary Tests

Potassium is usually ordered along with other electrolytes, including sodium, chloride, phosphate, and magnesium. This is important for many reasons. For example, low magnesium levels are common, and when low, magnesium must be replaced in order for potassium replacement to be effective. Kidney function tests are important if potassium levels are abnormal.

Risks and Contraindications

Since potassium is a simple blood test, there are few risks other than bruising related to the blood draw. It is important to note, however, that the potassium level that is returned may not reflect what is really happening in the body (the total body stores of potassium), and lab errors sometimes occur. Since potassium is mostly present inside the cells, if the blood cells are damaged during blood draw or transportation, the potassium level may be spuriously high. A repeat blood draw is usually indicated in these circumstances.

Before The Test

Before having your potassium test, your doctor will explain the purpose for this as well as other tests recommended. It is important that your doctor is aware of your medical history, especially any history of kidney disease or abnormal potassium levels in the past. If you have had recent lab tests done at outside clinics or hospitals, obtaining these records is helpful in order to make comparisons.

Some doctors recommend fasting for several hours prior to the blood test, especially if other tests such as cholesterol levels are being drawn at the same time.

Timing

A potassium test is frequently done at the same time as a clinic visit, and results may be available during your visit. In other cases, your doctor may call you later with your results. As with many other lab tests, it's helpful to ask your doctor both your level of potassium and the normal range, rather than simply whether it's normal, high, or low.

Location

A potassium blood test may be drawn in the hospital and many clinics. In a clinic setting, your blood may be drawn in an exam room, or you may be asked to go to a special area where blood tests are done.

What to Wear

It's helpful to wear a short-sleeve shirt or a long-sleeve shirt with loose sleeves that can easily be rolled up.

Food and Drink

While many labs do not have food or water restrictions prior to a potassium blood test, drinking large amounts just prior to your test may interfere with your results.

Cost and Health Insurance

A potassium blood test is relatively inexpensive, and is covered by insurance for many medical conditions.

What to Bring

It's important to bring your insurance card to your visit, as well as any recent lab tests from outside clinics or hospitals.

During the Test

When you arrive for your blood test, the technician will make sure you are comfortably seated in a chair. They will inspect your arm for accessible veins, and then cleanse the site where blood will be drawn with alcohol. A tourniquet may be applied to make the vein more visible.

While some technicians recommend fist clenching as a method to make veins more visible, this can result in falsely elevated potassium levels and should be avoided. Prolonged use of a tourniquet can also falsely elevate levels.

Fist clenching should be avoided as it can result in errors in your potassium level.

When the technician is ready, they will let you know that they are placing the needle in your arm and you may feel a sharp prick as it enters your skin. If you are bothered by blood draws or the site of blood it can be helpful to look at something else during the procedure. This discomfort is usually transient, though the procedure may sometimes need to be repeated more than once to obtain a sample.

The needle will be kept in place, attached to a test tube, and sometimes additional test tubes are placed to obtain further lab tests. You may feel some pressure while the needle remains in your arm.

When the technician has obtained the samples, they will remove the needle and cover the site with gauze or a bandage. You will be asked to hold pressure over the site for a few minutes to help stop any bleeding and reduce the chance of bruising. The entire process usually requires only a few minutes.

After the Test

After a few hours, you can usually remove the gauze or bandage that was placed on your arm. Some people may experience bruising, especially if more than one attempt is needed to obtain blood. People who have bleeding disorders or are taking blood thinners are also more likely to experience bruising.

Your doctor will let you know when you can expect to get your results. This may be as quickly as a few minutes following your blood draw or up to a week or more, depending on the lab and the urgency of the results.

Interpreting Results

When interpreting your results, it's important to understand that the normal range of potassium can vary in individuals and in different situations. Serum potassium levels can vary with the time of day, especially in people with kidney disorders. Levels tend to be higher in pregnancy, and lower in Asians and blacks than in whites.

Lower average potassium levels in blacks are actually thought to be one reason why the incidence of type II diabetes is higher in blacks.

Reference Range

The results are reported in milli-equivalents per liter (mEq/l). The reference range for a normal potassium can vary somewhat between labs, but is most often in the range of:

  • 3.5 mEq/l to 5.0 mEq/l in adults
  • 3.4 mEq/l to 4.7 mEq/l in children
  • 4.1 mEq/l to 5.3 mEq/l in infants
  • 3.9 mEq/l to 5.9 m Eq/l in newborns

A high potassium (hyperkalemia) is considered to be a potassium greater than 5.0 mEq/l (or slightly higher depending on the lab). Levels higher than 5.5 mEq/l are considered very high, and levels over 6.5 mEq/l can be life-threatening in adults. In newborns, levels greater than 8.0 mEq/l are considered critical.

A low potassium (hypokalemia) is considered to be a potassium less than 3.5 mEq/l. Levels less than 2.5 mEq/l are very serious.

Low Potassium (Hypokalemia)

There are three primary mechanisms that may result in a low potassium level in the blood:

  • A low intake of potassium in the diet (this is uncommon)
  • Increased potassium loss from the body (either via the kidneys (often due to medications), gastrointestinal tract, or via sweating (rare).
  • Shift of potassium from the bloodstream into cells (with insulin, when the pH of the blood is low (metabolic acidosis), when stress hormones are released, or with periodic paralysis

Some potential causes of a low potassium level include:

  • Vomiting or diarrhea
  • Dehydration
  • Medications, including some diuretics such as Lasix (furosemide), laxatives, insulin, glucocorticoids, penicillin, and acetaminophen (with overdoses)
  • Trauma
  • Increased aldosterone due to primary hyperaldosteronism, Cushing's syndrome, an excessive intake of European licorice, and more
  • Some kidney disorders, (chronic kidney disease (CKD) is the most common cause of chronic low potassium levels)
  • Magnesium deficiency
  • Barium poisoning
  • Uncommon genetic disorders such as Liddle syndrome, hypokalemic periodic paralysis, Bartter syndrome, or Gitelman syndrome
  • Low intake of potassium combined with high sodium intake (uncommon)
  • Factors such as chronic stress and alcoholism may also contribute

Symptoms are not usually present unless the potassium level drops below 3.0 mEq/l, and may include muscle cramps and weakness, fatigue, constipation, and when severe, paralysis or rhabdomyolysis. Seizures may also occur.

Treatment is usually accomplished with oral or intravenous potassium. When hypokalemia is chronic, treatment is often needed long after levels return to normal as total body stores may be very low despite blood levels appearing normal. Dietary potassium (eating foods rich in potassium) is not enough to improve a low potassium level due to diarrhea or diuretics.

High Potassium (Hyperkalemia)

Spurious high potassium levels (errors) may result from fist clenching during a blood draw, when hemolysis occurs in the sample, or in people who have very high white blood cell or platelet counts.

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An Overview of Hyperkalemia

There are also three primary mechanisms that can result in a potassium level that is truly too high (hyperkalemia). These include:

  • Increased intake of potassium
  • Decreased potassium excretion by the kidneys (often related to medications or lack of insulin)
  • Shift of potassium from cells into the bloodstream

Potential causes of an elevated potassium level include:

  • Kidney disease (usually with acute renal failure rather than chronic kidney disease)
  • Type I diabetes (lack of insulin)
  • Metabolic acidosis
  • Physical stress (trauma, burns, infections, dehydration)
  • Medications such as potassium-sparing diuretics, angiotensin converting enzyme inhibitors (ACE inhibitors) such as Zestril (lisinopril), angiotensin receptor blockers (ARBs), direct renin inhibitors, aldosterone antagonists, calcium channel blockers, beta-blockers, non-steroidal antiinflammatory drugs such as ibuprofen (NSAIDs and hyperkalemia), digitalis, calcineurin inhibitors, proton pump inhibitors (such as omeprazole), heparin, cyclosporine, trimethoprin, mannitol, and pentamidine
  • Blood transfusion
  • Hypoaldosteronism (such as with Addison's disease)
  • Tumor lysis syndrome (break down of cells due to cancer treatment)
  • Cirrhosis
  • Heart failure
  • Hemolytic anemia
  • Excessive intake via diet, supplements, or salt substitutes (uncommon unless other factors leading to high potassium levels are also present)
  • Some uncommon genetic disorders such as familial periodic paralysis
  • Eating disorders such as bulimia

Symptoms can be very serious and can range from muscle weakness to paralysis, and from palpitations to sudden death (due to dangerous heart rhythms). The combination of severe hyperkalemia and abnormal changes on an EKG is a medical emergency.

Treatment varies depending on the severity and may include insulin when needed, sodium bicarbonate, intravenous calcium, and dialysis when very serious. Long term treatment may include restricting potassium in the diet (low potassium diet), potassium wasting diuretics, medications that bind potassium, and more.

Further Testing

if abnormalities are not severe, and if an obvious cause is not present, the first step may be to simply repeat the test. As noted, errors in potassium can be caused by a number of factors including fist clenching during the blood draw or the prolonged use of a tourniquet. If a high white blood cell or platelet count are present, a plasma potassium (rather than whole blood sample) may be done.

If an abnormal potassium is present, kidney function tests as well as a glucose level should always be done. Other electrolytes (such as sodium) should be evaluated as they may affect treatment, for example, a magnesium test should be done as magnesium deficiency must be treated for the treatment of potassium deficiency to be effective. Evaluation of acid-base balance in the body is also important. A complete blood count may be done to rule out causes such as hemolytic anemia and to look for an elevated white blood cell or platelet count.

If an obvious cause for hypokalemia is not present, a common next step is to check the potassium level in the urine (either with a random urine sample or sometimes with a 24-hour urine sample). If the level of potassium in the urine is low, causes such as loss from the gastrointestinal tract or a shift of potassium into cells may be the cause. If the potassium level in the urine is high, the cause is likely related to kidney disease. Further tests may be indicated based on other lab studies, such as evaluation of aldosterone and much more.

Follow-Up

Follow-up will depend on many factors including the reason why the test was performed in the first place. If your levels are abnormal, it's important to talk to your doctor about the reasons why, any further testing that is indicated, and when you should have a repeat potassium test. It's helpful to write down any specific instructions or follow-up appointments.

Some people request copies of their blood work to keep their own records. This can be particularly helpful if you receive care from different doctors at different locations, or if you will be traveling.

You may wish to ask specific questions about what you can do yourself in regard to an abnormal potassium level. If you have a low potassium level, it's important to understand that eating bananas alone is unlikely to be an effective treatment. But if you have a high potassium level, strict attention to a low potassium diet may be very important.

Since abnormal (and when severe, potentially life-threatening) arrhythmias are of concern with high potassium levels, people who have a history or are at risk for hyperkalemia should familiarize themselves with the symptoms of abnormal heart rhythms.

A Word From Verywell

It can be frightening to learn about the potential causes for an abnormal potassium level, and this can be compounded if you and your doctor are uncertain of the cause. Asking a lot of questions, and being an active participant in your care can help you stay in the driver's seat of your health care and make sure you get the care you deserve.

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