30 October 20269 min read

High Potassium on a Blood Test: What It Means

Medical Review

Medically Reviewed by Dr. Pedro Yelamo Vera, MD

Licensed Physician No. 484814280 · Official Medical College of Bizkaia (Spain)Last reviewed: 31 October 2026

High potassium on a blood test: normal ranges, UK and US cut-offs, false results from the sample, common causes and when to call 112.

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High potassium on a blood test is often a mild rise, or a false result caused by the blood sample. A doctor usually repeats the test before doing anything else. When it is real, the commonest causes are the kidneys and some medicines. A level of 6.0 mmol/L or more needs urgent review, and 6.5 or more is an emergency.

TL;DR

Typical normal range: about 3.5 to 5.0 or 5.2 mmol/L. Each lab sets its own.
UK kidney guidance: mild 5.5 to 5.9, moderate 6.0 to 6.4, severe 6.5 or more.
Many high results are false: a damaged sample or a slow trip to the lab can raise the reading.
True high potassium mostly comes from kidney problems or medicines such as enalapril, spironolactone or ibuprofen.
Palpitations, chest pain, breathlessness or sudden weakness with a high result: call 112.

Dr. Pedro Yelamo Vera is a GP licensed in Spain (Colegiado No. 484814280).

For a medical emergency, call 112 or go to the nearest local emergency department. Do not wait for a video consultation or an online reply.

When is high potassium an emergency?

Potassium helps your nerves and muscles work, including the heart. Mayo Clinic warns that sudden or severe high potassium can be life-threatening. It can upset the heart rhythm, often with no warning symptoms.

Call 112 or go to urgencias if:

Your result is 6.5 mmol/L or more, even if you feel well.
You have a high result and palpitations, a slow or irregular heartbeat, chest pain, shortness of breath, fainting or sudden weakness in your arms or legs.

See a doctor today, without waiting, if the result is 6.0 to 6.4 mmol/L. UK guidance advises a repeat test within a day at this level. Do the same for a smaller rise if you are unwell, vomiting or not passing much urine. A sudden kidney problem may be the cause.

What is a normal potassium level?

Mayo Clinic gives a healthy range of 3.6 to 5.2 mmol/L. The US National Kidney Foundation says 3.5 to 5.0, depending on the lab. Always read the range printed next to your result.

Where the line for "high" sits also differs. Many US sources call anything above 5.0 or 5.2 high. UK guidance from the UK Kidney Association uses 5.5 mmol/L or more, a cut-off it takes from the European Resuscitation Council. A result of 5.3 may carry an "H" on your report but still be close to normal.

On a Spanish report, look for potasio or K. It is often part of the ionograma, with sodium (sodio) and chloride (cloro). Spanish labs use mmol/L or mEq/L. For potassium these are the same number.

How serious is my result? Mild, moderate and severe

The UK Kidney Association groups adults like this:

Mild, 5.5 to 5.9 mmol/L. If unexpected, repeat the test within 3 days, or as soon as you can.
Moderate, 6.0 to 6.4 mmol/L. Repeat it within 1 day.
Severe, 6.5 mmol/L or more. Go to hospital for assessment and treatment.

The speed of the rise matters too. A fast jump can be more risky than a steady, higher level. People with long-term kidney disease sometimes live with a level a little above normal, under their kidney team's care.

Can a high potassium result be wrong?

Yes, quite often. This is called pseudohyperkalaemia. Potassium leaks out of blood cells into the sample, so the number is high but the blood in your body is normal. Causes include:

Damaged red cells in the sample. This is called haemolysis. A Spanish report may say hemólisis or muestra hemolizada.
A long wait before the sample is processed. A hospital guide for UK GPs flags delays over 6 hours.
A sample kept in the fridge before it is spun down.
Clenching your fist or a tight band left on the arm for a long time.
Very high platelet or white cell counts.

So a doctor will usually repeat the test, taken carefully and sent straight to the lab. A false result needs no treatment.

Got a high potassium result and not sure how worried to be?

You can book a video visit with a GP licensed in Spain, in English or Spanish. He reads your whole report, checks your kidney results and medicines, and tells you whether you need a repeat test, more checks or urgent care.

Get your blood test reviewed

What causes high potassium?

Mayo Clinic says the commonest cause of a true high level is the kidneys. Healthy kidneys remove extra potassium in urine. Other causes:

Medicines. ACE inhibitors such as enalapril or ramipril, ARBs such as losartan, spironolactone, anti-inflammatory painkillers such as ibuprofen, the antibiotic trimethoprim (also in Septrin), beta blockers, and potassium supplements.
Salt substitutes. Many "low-sodium" salts are made with potassium chloride. The National Kidney Foundation advises people at risk to avoid them.
Diabetes that is poorly controlled, especially type 1.
Addison's disease, where the adrenal glands make too little hormone.
Dehydration, burns, crush injuries or muscle breakdown, which release potassium from cells.

Food alone rarely causes it. MedlinePlus lists a diet very high in potassium as a cause, but says it is not common.

What are the symptoms of high potassium?

Often there are none. Most high results are found on a routine test. When symptoms appear, Mayo Clinic and MedlinePlus list:

Muscle weakness or tiredness.
Numbness or tingling in the arms or legs.
Nausea or vomiting.
Palpitations or an irregular heartbeat.
Shortness of breath or chest pain.

The last two groups are warning signs. With a high result, call 112.

What happens next?

Repeat test. Taken without a long tourniquet or fist clenching, and processed promptly.
Kidney check. Creatinine, eGFR, urea and blood sugar.
Medicine review. The doctor may pause or change a medicine. Do not stop a prescribed medicine on your own.
An ECG (heart tracing). UK GP guidance advises one for a confirmed level of 6.0 or more.
Diet advice, only if you have kidney disease and other causes are dealt with. UK guidance advises a kidney dietitian for this.

In hospital, severe high potassium is treated with medicines that protect the heart and move potassium back into cells. Some people with kidney disease or heart failure take a potassium binder long term. It removes potassium through the gut.

Low potassium (below 3.5) also matters, as it can affect the heart and muscles. Mention it to a doctor too.

Talking it through with a doctor by video

Dr. Yelamo is a GP licensed in Spain. He sees adults, children and pregnant patients by video, every day from 07:30 to 20:30 (Spain time). In the visit he will:

Read your full blood report, not just the potassium line.
Ask about every medicine, supplement and salt substitute you use.
Check your kidney results and any recent illness, vomiting or diarrhoea.
Tell you whether you need a repeat test, an ECG, a specialist or emergency care.

He does not promise any test or prescription before the visit. Where clinically appropriate, following a genuine evaluation. Treatment and prescriptions are issued solely at the doctor's discretion based on evidence-based clinical guidelines.

A prescription is never guaranteed: it is issued only where clinically appropriate after a genuine evaluation, as a private electronic prescription under Spanish law. This service is for urgent, non-emergency care. In an emergency, call 112.

See how the Urgent Care video visit works. It lasts 15 minutes and costs €50.

This guide is general information. It does not replace a consultation with your doctor.

Frequently asked questions

Is a potassium of 5.6 dangerous?

Usually not on its own. UK guidance calls 5.5 to 5.9 mmol/L mild and advises a repeat test within a few days. Seek help at once if you have palpitations, chest pain or sudden weakness.

What potassium level is an emergency?

UK kidney guidance treats 6.5 mmol/L or more as severe, needing hospital care. A level of 6.0 to 6.4 needs a repeat test and a doctor's review within a day.

Can a blood test show high potassium by mistake?

Yes. Damaged blood cells in the sample, a slow trip to the lab or clenching your fist can all give a falsely high result. That is why a doctor usually repeats the test.

Which medicines raise potassium?

Common ones are ACE inhibitors such as enalapril, ARBs, spironolactone, anti-inflammatory painkillers such as ibuprofen, trimethoprim and potassium supplements. Do not stop a prescribed medicine without advice.

Should I stop eating bananas?

Not unless a doctor or dietitian tells you to. With healthy kidneys, food alone rarely causes high potassium. Diet changes are for people with kidney disease, with expert advice.

Is mEq/L the same as mmol/L for potassium?

Yes. For potassium the number is the same in both units, so 5.5 mEq/L equals 5.5 mmol/L.

Sources

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Medical Citations & Sources

This content is written and reviewed by a licensed physician with reference to the following official health authorities.