Potassium
Serum Potassium
What this test measures
Potassium is a mineral essential for your heart, muscles, and nerves to work properly. Your kidneys control potassium levels. Abnormal levels can affect your heart rhythm.
Understanding the numbers
Typical range: 3.5-5.3 mmol/L
Typical UK range. Some labs use 3.5-5.0 mmol/L. Potassium must stay within a narrow range - both high and low levels can be dangerous.
- Severe low: 2.0–2.5 mmol/L
- Low: 2.5–3.5 mmol/L
- Normal: 3.5–5.3 mmol/L
- Raised: 5.3–6.0 mmol/L
- Severe high: 6.0–8.0 mmol/L
Key numbers
- 3.5 mmol/L — Below = hypokalaemia
- 5.3 mmol/L — Above = hyperkalaemia
- 6.0 mmol/L — Above = urgent — ECG and hospital treatment
- 6.5 mmol/L — Critical — same-day treatment always
UK Renal Association guidelines stratify hyperkalaemia as mild (5.5-5.9), moderate (6.0-6.4) and severe (≥6.5 mmol/L). Severe hyperkalaemia causes cardiac arrhythmias — treatment must not be delayed. Haemolysis (broken sample) can falsely raise potassium — if the result doesn't fit the patient, repeat.
Ranges can vary slightly between laboratories — your report may show slightly different numbers. This page does not read or interpret your result; your GP looks at it alongside your history and other tests.
What a higher-than-normal result can mean (Hyperkalaemia)
- Can affect your heart rhythm
- Often caused by kidney problems
- Some medicines can raise potassium (ACE inhibitors, potassium supplements)
- May need urgent attention if very high
When not to wait
Very high or very low potassium can cause dangerous heart rhythms.
- Palpitations or a racing / irregular heartbeat
- Muscle weakness, especially in the legs
- Feeling about to collapse or faint
- Chest pain or extreme breathlessness
- Tingling around the mouth or in the fingers and toes
- Known potassium above 6.0 or below 3.0 mmol/L
Call NHS 111 for urgent advice, or 999 if someone is seriously unwell.
Questions you could ask your GP
- Is my potassium level safe?
- Could any of my medicines be affecting this?
- Do I need to change my diet?
- How soon should this be rechecked?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
Hyperkalaemia on ACE + spironolactone
Anil has heart failure on ramipril, spironolactone and bumetanide. Bloods showed potassium 6.4, creatinine up, and eGFR dropped to 28.
What the GP said: “Your kidneys have taken a hit and the three combined medicines are now pushing potassium too high. Stop the spironolactone today and go to A&E — they'll do an ECG and may need to lower the potassium with medication.”
What happened next: ECG was normal. With stopping spironolactone and a short course of patiromer, potassium was 5.1 at 48 hours.
ACE inhibitors, ARBs, spironolactone and trimethoprim all raise potassium. The risk goes up sharply if the kidneys slip. When in doubt, check an ECG and stop the culprit.
Hypokalaemia on furosemide
Daniela has chronic heart failure on furosemide 80 mg. She had muscle cramps and tiredness. Potassium 2.9.
What the GP said: “Furosemide pushes potassium out in the urine. We'll add a potassium-sparing diuretic and give you a short course of oral potassium. Recheck in a week.”
What happened next: With spironolactone and a week of potassium tablets, her potassium rose to 4.2.
Loop diuretics are a common cause of low potassium. Combining with a potassium-sparing diuretic or supplement usually corrects it. Low potassium with digoxin in particular is dangerous — it raises arrhythmia risk.
False hyperkalaemia from haemolysis
Harley was well. His potassium came back at 6.2 — 'haemolysed sample' was noted. ECG was normal.
What the GP said: “Your sample was damaged on the way to the lab and broken red cells release potassium, making it look high. We need a fresh, carefully drawn sample.”
What happened next: Repeat potassium from a cleanly drawn sample was 4.3 — normal.
Haemolysed samples falsely raise potassium. If the result doesn't fit the patient, always repeat before treating. Look for a 'haemolysed' note on the lab comment.
Low K and Mg — correct both
Kamal had vomiting and diarrhoea for five days and was taking omeprazole. K 2.6, Mg 0.5.
What the GP said: “You've lost potassium from the gut, and low magnesium makes it hard to replace potassium. We'll replace both — IV potassium and IV magnesium — and recheck.”
What happened next: Both corrected over 24 hours and stabilised.
If potassium won't come up, check magnesium. Low magnesium makes the kidneys continue to dump potassium — replacing potassium alone often fails.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.