What Does a Low Potassium Blood Test Mean?

A low potassium blood test means the level of potassium in your blood is below the laboratory's reference range. This is called hypokalaemia (hypokalemia in US spelling). It is most often caused by losing potassium through the kidneys or gut, for example from diuretic medicines, vomiting, diarrhoea or laxatives.

Potassium is an electrolyte that helps nerves, muscles and the heart work properly, so the size of the drop and any symptoms matter. This guide explains the result. It cannot tell you how serious your own level is or what treatment you need.

Nearly all of the body's potassium is stored inside cells, and the kidneys remove any extra in urine. A blood test measures only the small amount in the blood, which is why changes in fluid, medicines and kidney handling can shift the result quite quickly.

MedlinePlus explains that a large drop in potassium can cause abnormal heart rhythms, especially in people with heart disease, and that very low levels can be life-threatening. Severe cases can also cause dangerous muscle weakness.

Call your local emergency number or go to an emergency department if you have any of the following, whether or not you know your potassium level. Do not drive yourself.

• Palpitations that do not settle, or palpitations with chest pain, shortness of breath or feeling faint.
• Fainting or near-fainting.
• Sudden or severe muscle weakness, or difficulty moving your arms or legs.

Contact a clinician the same day if you have been vomiting or have had heavy diarrhoea, or if you take a diuretic and notice possible symptoms of low potassium. Also mention any known heart condition, because MedlinePlus notes that people with heart disease are more at risk of rhythm problems from low potassium.

• Muscle cramps, twitching or weakness.
• Unusual tiredness.
• Constipation or nausea.
• Tingling or numbness.

MedlinePlus lists several common causes. Often more than one is present at the same time.

Healthdirect Australia notes that hypokalaemia is not common in the general population. When it is found, the cause is often clear from recent illness or a medicine list. If no obvious cause is found, or the level keeps falling despite treatment, a clinician may look for less common causes such as a hormone condition.

• Diuretics ("water pills"), which increase the amount of urine and the potassium lost in it.
• Vomiting or diarrhoea, including from a stomach bug.
• Using too many laxatives.
• Heavy sweating.
• Low magnesium.
• Eating disorders, including bulimia.
• Alcohol use disorder and some kidney conditions.
• Less commonly, hormone conditions such as aldosteronism (also called hyperaldosteronism) or Cushing syndrome, or rare inherited conditions.
• A diet very low in potassium, which on its own is an uncommon cause.

Low magnesium is one of the listed causes of low potassium. MedlinePlus notes that a magnesium test can help find the cause of abnormal levels of other minerals, including potassium, and that low potassium is one reason to monitor magnesium.

Some of the same things lower both, including diuretics, long-lasting diarrhoea and alcohol use disorder. MedlinePlus also lists proton pump inhibitors, a type of stomach acid medicine, as a cause of low magnesium. If your potassium is low, it is reasonable to ask whether magnesium was measured too.

Your clinician may repeat the potassium test and check other blood results such as magnesium, calcium, sodium, glucose and kidney function. Depending on the situation, thyroid or aldosterone tests may be considered. An ECG (heart tracing) may be done to look for effects on heart rhythm.

It helps your clinician to know about any recent vomiting, diarrhoea or heavy sweating, how much you have been eating and drinking, and every medicine you take, including laxatives bought without a prescription. Laxative use is easy to leave off a medicine list, but it is one of the recognised causes.

Reference ranges differ between laboratories, so compare your result with the range printed on your own report. If your potassium was high on another test rather than low, see our separate guide on a high potassium blood test and possible sample errors.

Treatment depends on the cause and how low the level is. MedlinePlus says mild cases are usually treated with potassium taken by mouth, and severe cases may need potassium through a drip in hospital. If you need a diuretic, a clinician may change the type or add potassium.

Treating the cause matters as much as replacing potassium. For example, the level may recover once vomiting or diarrhoea has stopped, or after a medicine is reviewed. If magnesium is also low, potassium can be harder to correct until magnesium is addressed, so both may be rechecked together.

MedlinePlus notes that potassium-rich foods, such as bananas, spinach, avocados and salmon, can help prevent and treat low levels. Ask whether diet changes are suitable for you, especially if you have kidney disease or take medicines that raise potassium.

Do not start potassium supplements, change a diuretic dose or stop a prescribed medicine on your own. Too much potassium can also be harmful, so any treatment should be guided and rechecked by a clinician.

Bring the report with the units, reference range and any earlier results, and a list of all your medicines, including laxatives and supplements.

• How low is my potassium, and does this level need treatment today?
• What do you think caused it? Could any of my medicines be involved?
• Was my magnesium checked, and was it normal?
• Do I need an ECG?
• When should my potassium be rechecked, and who will tell me the result?
• Which symptoms should make me seek urgent care before then?

On SameCase, patients and verified doctors share de-identified cases. Seeing how others with low potassium had their medicines, magnesium and follow-up tests discussed can help you list your own questions before an appointment. You might notice, for example, which questions others asked about diuretics or laxatives, or how often their levels were rechecked.

Similar cases cannot tell you what is causing your result or how urgent it is. Use them to prepare, and rely on your own clinician for decisions.