Is my potassium high?
Yes, if it’s above 5.3. That’s the top of normal at NHS labs like South Tees. But one unexpected high is often the blood sample, not your blood. A clenched fist can push it up.
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Is my potassium level high?
Yes, if it’s above 5.3 mmol/L. South Tees NHS lab gives a normal adult range of 3.5 to 5.3, and North Bristol uses the same. Ranges for children differ, and your lab prints its own range next to your result.
How far above matters. NHS lab guides for GPs call 5.5 to 5.9 mild and 6.0 to 6.4 moderate. At 6.5 or more it’s severe and potentially life threatening.
- 5.3 mmol/L or belowNormal for adults at South Tees and North Bristol.
- Above 5.3 to just under 6 mmol/LAbove normal. 5.5 to 5.9 is mild and needs review.
- 6 to just under 6.5 mmol/LModerate. Needs urgent review or treatment.
- 6.5 mmol/L or aboveSevere. Needs emergency treatment.
Sources: South Tees NHS; North Bristol NHS; RUH Bath
Think of the range as a speed limit. A 5.4 is just over it, below the 5.5 that NHS lab guides for GPs call mild. A 6.8 is far over and needs emergency care, because severe high potassium can cause an irregular heartbeat.
Check which band your number sits in. Then check whether it could be a false result.
Could my high potassium be a false result?
Yes, especially if it was a one-off or unexpected. Only about 2% of your potassium sits outside your cells. The rest is packed inside them.
So if blood cells break while your blood is taken, the tube can read high when the level in your body is normal. That’s called a false high (pseudohyperkalaemia).
| What happened | Example |
|---|---|
| Problems during the draw | Blood cells breaking, or the arm band left on too long. |
| The sample was slow to reach the lab | Especially a delay of more than 6 hours. |
| The sample was chilled too early | Put in a fridge before the lab separated it. |
| It mixed with another tube | Fluid from a blood count tube. Labs usually spot this. |
| Very high platelets or white cells | These can push up the reading in the tube. |
Sources: RUH Bath; North Bristol NHS
Your fist can do it too. In a small study of 16 healthy people, clenching the fist 6 times and holding it during the draw raised potassium by 13.4%.
RUH Bath says a false high is a common cause when a rise is one-off or unexpected. That’s especially so with no symptoms, no kidney disease and a normal heart trace.
At your repeat test, ask whether you need to make a fist. More tips in how to prepare for a blood test.
What causes high potassium in the blood?
When a high is real, your GP usually checks your kidneys first. People with kidney disease can get high potassium because their kidneys do not work properly.
| Cause | Examples |
|---|---|
| Kidney problems | Sudden kidney injury or long-term kidney disease. |
| Medicines | Some blood pressure tablets (ACE inhibitors, ARBs), water tablets that keep potassium in (spironolactone, amiloride), anti-inflammatory painkillers (NSAIDs), the antibiotic trimethoprim, the blood thinner heparin, and heart medicines (beta-blockers, digoxin). |
| Low aldosterone, a salt-balancing hormone | For example, Addison’s disease. |
| Potassium leaving the cells | Diabetic ketoacidosis (DKA), a crush injury or other muscle breakdown. |
| Too much potassium in | Diet (usually only with kidney problems), potassium supplements, or laxatives that contain potassium. |
Sources: RUH Bath; North Bristol NHS
DKA can happen in people with diabetes. A lack of insulin lets harmful ketones build up, and it needs urgent hospital treatment.
On one of these medicines? Never stop or change a prescribed medicine without your prescriber. Bring a full list, supplements included, to your GP.
Your kidney numbers usually come on the same report. See U&E results explained and the eGFR blood test.
What will my GP do after a high result?
Most often, a repeat test. How soon depends on your level and whether the result was expected.
| Potassium (mmol/L) | If you’re well | If unexpected |
|---|---|---|
| 5.5 to 5.9 | Within 14 days | Within 3 days |
| 6.0 to 6.4 | By the next working day | Within 24 hours |
| 6.5 or more | Hospital assessment straight away | Hospital assessment straight away |
Source: North Bristol NHS
Areas differ. RUH Bath repeats a mild high within 1 week.
Above 6.0, North Bristol says you should have a heart trace (ECG), a quick check of your heart’s rhythm. Your GP may also check your kidneys, blood sugar and medicines.
Speed matters as well as size. A rise of more than 0.5 in 6 to 12 hours may matter more than the number itself.
Ask your GP when your repeat is due and book it.
Should I cut down on potassium?
Ask first. Diet usually only causes a high alongside kidney problems. Your GP may ask about foods like bananas, nuts, dried fruit and avocado.
With kidney disease, if blood tests show high potassium and phosphate, you may be referred to a dietitian. They’ll advise how much potassium to include.
Supplements are different. The NHS says older people should not take potassium supplements unless a doctor advises it. People with kidney disease should avoid them.
More in potassium supplements and potassium salt swaps.
Before your repeat potassium test
- Write down your result and the range printed next to it.
- Book the repeat test your GP asks for, even if you feel well.
- Ask whether you need to make a fist while your blood is taken.
- List every medicine and supplement you take, including laxatives.
- Never stop or change a prescribed medicine without your prescriber.
- Ask your GP before cutting foods like bananas, nuts or dried fruit.
Sources: RUH Bath; North Bristol NHS; NHS; Fist study
When to speak to a GP
High potassium can cause muscle weakness, stiffness and tiredness. Get help if:
- You have heart palpitations that don’t go away, or with chest pain, shortness of breath, feeling faint or fainting. Call 999 or go to A&E. If they’ve stopped, ask your GP for an urgent appointment, or call 111.
- Your potassium is 6.5 or more. The lab phones these to your GP or out-of-hours service. North Bristol sends you to hospital for assessment straight away.
- Your potassium is 6.0 to 6.4. North Bristol repeats it by the next working day if you’re well, or within 24 hours if unexpected.
- You have a high result and feel very tired, sick, weak or have pins and needles. GPs check for these symptoms.
- You have kidney disease and take potassium supplements, or medicines for blood pressure or heart failure. The NHS says to talk to a GP about them.
Potassium usually comes as part of a kidney test. See the kidney function test.