What Does a High Potassium Mean?
A doctor explains what a high potassium result means, why a sample problem is the commonest cause of a false high, and when the level becomes an emergency.
The short version
- Potassium controls the electrical signaling of nerves, muscle and especially the heart, so a genuinely high level matters even when you feel completely well.
- Normal is about 3.5 to 5.0 mmol/L (the same as mEq/L); the single commonest reason for a high reading is a sample problem where red cells burst during collection, not real illness.
- When it is real, the usual causes are reduced kidney function and medicines, particularly ACE inhibitors, ARBs, spironolactone and potassium supplements.
See a doctor promptly if
These are the signs that change this from something to read about into something to act on.
- A confirmed potassium above about 6.0 mmol/L, which needs same-day medical assessment
- High potassium with palpitations, a slow or irregular pulse, chest pain or feeling faint, which needs emergency care
- High potassium with muscle weakness, especially if it is spreading or worsening
- High potassium in someone with known kidney disease or on dialysis
Potassium runs the electrical activity of nerve and muscle, including heart muscle, so the body keeps it in a narrow band. A high potassium, called hyperkalaemia, matters because a severely raised level can disturb heart rhythm, sometimes before you feel anything at all. Equally important: the most common reason for a high potassium on a report is a problem with the blood sample rather than with the person.
Normal is roughly 3.5 to 5.0 mmol/L, and mmol/L and mEq/L are identical numbers for potassium.
First, is it real?#
Red blood cells contain far more potassium than the plasma around them. If they rupture between the needle and the analyser, potassium leaks out and the result is falsely high. This happens with a difficult draw, a very fine needle, a tourniquet left on too long, fist clenching, or a delay before the sample reaches the laboratory.
Many laboratories flag this as a haemolysed sample. Very high white cell or platelet counts can also release potassium after the tube is drawn. This is why a first high potassium in a well person with normal kidneys is usually repeated before anything is done.
What the level means#
- 5.1 to 5.5 mmol/L: mild, usually needs a repeat and a medicine review
- 5.6 to 5.9 mmol/L: moderate, assessed within days
- 6.0 mmol/L and above: significant, needs same-day assessment and usually an ECG
Symptoms are unreliable. Muscle weakness, tingling and palpitations can occur, but many people with a potassium of 6.2 feel entirely normal. This is one of the few results where the number is more trustworthy than how you feel.
The real causes#
Reduced kidney function is the biggest, because the kidneys are the main route of potassium excretion. Medicines come a close second, particularly ACE inhibitors, ARBs, spironolactone, potassium supplements and potassium based salt substitutes. Several of these are prescribed together in heart failure, which is why potassium is monitored in those patients.
Other causes include poorly controlled diabetes, adrenal insufficiency, dehydration, and any situation causing rapid tissue breakdown such as a crush injury, severe burns or extreme exertion.
What happens next#
Expect a repeat sample taken carefully, kidney function tests, glucose, and a review of every medicine and supplement you take, including anything bought over the counter. An ECG is done when the level is significantly raised, because the effect on the heart's electrical tracing guides urgency more than the number alone.
Management then depends on cause: correcting dehydration, adjusting or spacing medicines, treating diabetes better, and in the case of reduced kidney function, dietary advice from a renal dietitian. Acute treatment in hospital exists for high levels and works quickly.
Reading the panel as a whole#
Potassium is interpreted with sodium, urea, creatinine and eGFR on the same panel, and those numbers usually explain each other. The wider guide to understanding your blood test results walks through how that group is read.
Common questions
Why do labs ask to repeat a high potassium?
Which medicines raise potassium?
Can diet alone cause high potassium?
Sources
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