High Potassium (Hyperkalaemia)
Potassium keeps your heart and muscles firing normally. A high result on a routine test is often a false alarm caused by the sample itself, but a true high level can upset the heart rhythm, so it’s always rechecked promptly.
Quick answer: Potassium above about 5.2 mmol/L is high. Mild results of 5.5–5.9 are often from a haemolysed or delayed sample, or from medicines and kidney disease, and are repeated within days. 6.0–6.4 needs a repeat within a day. 6.5 or more is severe and needs hospital assessment straight away.
What the test measures
Almost all your potassium sits inside your cells, and only a little is in the blood. The kidneys keep the blood level in a narrow range. Because cells are so full of potassium, anything that damages blood cells in the tube releases potassium and makes the result look falsely high.
Typical adult ranges
- Normal: about 3.5–5.2 mmol/L. US labs report mEq/L, which is the same number.
- Mild hyperkalaemia: 5.5–5.9 mmol/L
- Moderate: 6.0–6.4 mmol/L
- Severe: 6.5 mmol/L or more
- These severity bands come from the UK Kidney Association guideline. Use the range on your own report.
False highs (pseudohyperkalaemia)
- Haemolysis: red cells burst during a difficult blood draw, with a tight tourniquet left on too long, or from clenching your fist. The lab often reports the sample as haemolysed.
- A delay getting the sample to the lab, especially over 6 hours, or a sample that’s been refrigerated before it was spun.
- Contamination from another tube’s preservative (EDTA), which labs usually detect.
- Very high platelet or white cell counts. See high platelets.
What a true high result usually means
Common causes:
- Kidney disease, long-term or a sudden drop in kidney function, often with dehydration. See low eGFR.
- Medicines: ACE inhibitors, sartans (ARBs), spironolactone and eplerenone, potassium supplements, trimethoprim, anti-inflammatories and heparin. See ACE inhibitors.
- Salt substitutes, which are often potassium chloride, especially with kidney disease.
- Uncontrolled diabetes: a lack of insulin lets potassium shift out of cells.
Less common but important:
- Addison’s disease (adrenal insufficiency), often with low sodium. See Addison’s disease.
- Muscle breakdown (rhabdomyolysis) after a crush injury, a long lie on the floor, extreme exercise or some drugs.
- Diabetic ketoacidosis and other severe acid build-up in the blood.
- Tumour lysis: cancer cells breaking down quickly after treatment.
Symptoms
- Often none, even at quite high levels, which is why the number matters.
- Muscle weakness, tiredness, nausea or tingling.
- Palpitations, a slow or irregular heartbeat, fainting or, at worst, cardiac arrest, sometimes without warning.
What happens next
- Repeat the test, with a fresh sample that reaches the lab quickly. Many mild results are normal on repeat.
- An ECG (heart tracing) if potassium is 6.0 or more, or with symptoms.
- Kidney function, glucose and a medicine review. Your doctor may pause or lower a medicine, but don’t stop prescribed medicines without advice.
- Timing (UK Kidney Association): mild results are repeated within about 3 days if unexpected; moderate results within 1 day; severe results need hospital assessment immediately.
- In hospital, treatment protects the heart and moves potassium back into cells, then removes it from the body.
- Longer term: adjusting medicines, a lower-potassium diet with a dietitian if you have kidney disease, and sometimes medicines that bind potassium in the gut.
Go to an emergency department now (call 000 in Australia, or your local emergency number, if you’re unwell) if a doctor tells you your potassium is 6.5 mmol/L or more: UK Kidney Association guidance is immediate hospital assessment. The same applies at any raised level with palpitations, a very slow or irregular pulse, chest pain, fainting or new muscle weakness. See a doctor the same day for a result of 6.0–6.4 mmol/L: it should be repeated within a day, usually with an ECG. Labs phone doctors about very high results, so if you get a call, act on it.
Practise reading results. Play Panelysis, the daily blood-test puzzle: a fictional patient, a real-looking panel, three guesses. More in medical games.
Common questions
What potassium level is dangerous?
Severe hyperkalaemia is 6.5 mmol/L or more and needs hospital assessment straight away, because it can cause dangerous heart rhythms. 6.0–6.4 is moderate and should be repeated within a day. Lower levels can still matter if they’re rising fast or causing symptoms.
Can a blood test show high potassium by mistake?
Yes, often. If red blood cells break in the tube, or the sample sits too long before reaching the lab, potassium leaks out and the result looks high. Very high platelet or white cell counts can do the same. A repeat sample usually sorts it out.
Is a potassium of 5.6 bad?
It’s mildly high. It’s often a sample problem, a medicine effect or a sign of reduced kidney function, and it’s usually repeated within a few days along with kidney tests. Your GP may review medicines such as ACE inhibitors or spironolactone.
Which medicines raise potassium?
Common ones include ACE inhibitors, sartans (ARBs), spironolactone, eplerenone, potassium supplements, trimethoprim, anti-inflammatories such as ibuprofen, and heparin. The risk is higher when several are combined, or with kidney disease or dehydration.
Can eating bananas cause high potassium?
Not if your kidneys work normally: they get rid of extra potassium from food. With kidney disease or certain medicines, high-potassium foods and potassium-based salt substitutes can matter, and a dietitian can help.
What are the symptoms of high potassium?
Often there are none. Some people get muscle weakness, tiredness, nausea or palpitations. Very high levels can cause a slow or irregular heartbeat or cardiac arrest, sometimes without warning, which is why high results are acted on quickly.
Evidence and more information
- Pathology Tests Explained (RCPA): Potassium
- UK Kidney Association: Clinical practice guideline, management of hyperkalaemia in adults (severity bands, repeat timing, community referral)
- Royal United Hospitals Bath NHS: Hyperkalaemia, a guide for GPs (spurious results, when to admit)
maladywise.com/blood-tests/high-potassium/