What is a normal potassium level?
3.5 to 5.3 mmol/L for adults at NHS labs like South Tees. But the tube can mislead. A clenched fist can push potassium up. A long, warm trip to the lab can pull it down.
On this page
What is a normal potassium level?
For adults, normal blood potassium is 3.5 to 5.3 mmol/L. That’s the range at South Tees and North Bristol NHS labs.
Children differ. South Tees uses 3.5 to 5.0 for ages 1 to 16. Your report prints your own lab’s range next to the result, so check that too.
- Below 3.5 mmol/LLow. Below 2.5 needs urgent checks.
- 3.5 up to 5.3 mmol/LNormal for adults at South Tees and North Bristol.
- Above 5.3 to just under 6.5 mmol/LHigh. 6.0 to 6.4 needs urgent review.
- 6.5 mmol/L or aboveSevere. Needs emergency treatment.
Sources: South Tees NHS; North Bristol NHS; RUH Bath; RUH Bath
How far outside matters. NHS lab guides for GPs treat 6.5 or more, or below 2.5, as urgent. The lab phones those results through.
Find your band. Then check whether the sample itself could be off.
What does a potassium blood test check?
It checks the potassium in your blood. Potassium is a mineral that helps control the balance of fluids in your body. It also helps your heart muscle work properly.
Most of it sits inside your cells. Only about 2% is outside them, and that small amount matters for your nerves, muscles and heart.
It usually comes as part of a urea and electrolytes (U&E) test, also called a renal panel. The NHS says this checks if certain substances in your blood may be causing your symptoms.
| Reason | What it can do to potassium |
|---|---|
| Diarrhoea or being sick a lot | Pull it down. This is the most common cause of a low. |
| Water tablets (diuretics) | Some pull it down. Others, like spironolactone, can push it up. |
| Kidney problems | Push it up, because the kidneys clear less. |
| Diabetic ketoacidosis (DKA), a serious diabetes problem | Push it up. |
| Some blood pressure tablets (ACE inhibitors, ARBs) | Push it up. |
Sources: South Tees NHS; RUH Bath; RUH Bath
North Bristol aims to report GP samples within 24 hours. See how long blood test results take for other routes.
Ask your GP which reason applies to you.
Could my potassium result be wrong?
Yes. Because nearly all your potassium sits inside cells, anything that breaks blood cells in the tube can push the reading up. That’s a false result: the tube, not your body.
| What happened | Which way it moves |
|---|---|
| You clenched your fist during the draw | Up |
| Blood cells broke, or the arm band stayed on too long | Up |
| The sample was slow to reach the lab, especially over 6 hours | Up |
| The sample went in a fridge before the lab processed it | Up |
| A slow trip to the lab in warm weather | Down |
| Very high white blood cells | Up or down |
Sources: RUH Bath; RUH Bath; Fist study
The clenched fist is real. 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 next test, ask whether you need to make a fist. More tips in how to prepare for a blood test.
What do low and high potassium mean?
Low is below 3.5. High is above 5.3. NHS lab guides for GPs then grade how far out it is.
| Potassium (mmol/L) | What the guide says |
|---|---|
| Below 2.5, or low with symptoms | Severe. Urgent checks with a heart trace (ECG). |
| 2.5 to 3.4 with no symptoms | Mild to moderate low. The GP looks for the cause, such as medicines or sickness and diarrhoea. |
| 5.5 to 5.9 | Mild high. Needs review. |
| 6.0 to 6.4 | Moderate high. Needs urgent review or treatment. |
| 6.5 or more | Severe and potentially life threatening. Needs emergency treatment. |
A low often causes no symptoms. When it does, they can include muscle cramps and pain, weakness, tiredness, palpitations, fainting and constipation. Low magnesium can cause low potassium, so ask whether your magnesium has been checked.
For a high, the GP may first check for a false result, then look at your kidneys and medicines. The high potassium guide covers repeat timing in detail.
Many of the causes are medicines. Never stop or change a prescribed medicine without your prescriber.
Do I need to change what I eat?
Not without advice. Adults aged 19 to 64 need 3,500mg of potassium a day, and the NHS says a varied diet should give you all you need.
Good sources include bananas, broccoli, parsnips, beans, nuts, fish and chicken. Food alone rarely causes a high. RUH Bath says diet usually only does it alongside kidney problems.
Supplements are different. The NHS says 3,700mg or less a day is unlikely to have obvious harmful effects. Older people should not take them unless a doctor advises it. More in potassium supplements.
Before your next potassium test
- Write down your result and the range printed next to it.
- Ask whether you need to make a fist while your blood is taken.
- List every medicine and supplement you take, including water tablets and laxatives.
- If your potassium is low, ask whether your magnesium has been checked.
- Ask your GP before cutting potassium foods or taking a potassium supplement.
- Never stop or change a prescribed medicine without your prescriber.
Sources: RUH Bath; RUH Bath; Fist study; NHS
When to speak to a GP
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, or below 2.5. The lab phones these through, and you may need urgent hospital care.
- 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 low result with cramps, weakness, palpitations or fainting. RUH Bath says this needs urgent checks with a heart trace.
Potassium sits next to your kidney numbers on the report. See U&E results explained.