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Potassium

Potassium is an electrolyte that is essential for your heart rhythm, muscle function and nerve signaling.

What is Potassium?

Potassium is the main electrolyte inside cells. Almost all of it sits intracellularly; only a small fraction circulates in the blood. The kidneys regulate the blood level tightly through the hormone aldosterone. Potassium is essential for the electrical conduction in heart and muscle cells. The normal serum range sits in a narrower window than most other blood markers. Even modest deviations can therefore be clinically significant. Always read potassium alongside sodium, creatinine, and eGFR.

Why is Potassium relevant?

Low potassium raises the risk of arrhythmias and causes muscle weakness, cramps, and fatigue. Diuretics (water pills) are the most common cause — they actively drive potassium loss. Prolonged diarrhoea, vomiting, or insufficient dietary intake can also cause a deficiency. High potassium slows electrical conduction in the heart. The most common cause is declining kidney function. Certain medications such as ACE inhibitors and some blood pressure treatments also raise potassium. Magnesium deficiency is an overlooked cause of persistently low potassium: without sufficient magnesium the kidneys spill potassium, making potassium supplementation ineffective until magnesium is restored.

Potassium high or low — what it means

Read potassium alongside sodium, creatinine, and eGFR. An unexpectedly high result: always rule out haemolysis first. If red blood cells lyse in the collection tube, intracellular potassium leaks out and inflates the reading. Repeat the test if the result does not fit the clinical picture. A persistently abnormal potassium confirmed on two measurements warrants investigating the cause. Muscle weakness or palpitations alongside an abnormal potassium call for prompt medical assessment. Also test magnesium if a low potassium does not rise with supplementation.

Potassium reference ranges

NormalSame for men and women3.5-5.0 mmol/L
Low (hypokalaemia)< 3.5 mmol/L
High (hyperkalaemia)> 5.0 mmol/L
Severely highEmergency — needs immediate medical assessment; ECG changes can appear from ~6.0 mmol/L≥ 6.5 mmol/L

Cutoffs vary slightly by lab and method; potassium has a narrower normal window than most markers. Haemolysis (cells breaking in the tube) can falsely raise the value.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.

Read about our scientific approach →Read the guide: Supplements →

Frequently asked questions

What is a normal potassium level?

Most Dutch labs use 3.5-5.0 mmol/L, the same for men and women. Potassium has a narrower normal window than many other markers, so even small deviations can matter. The exact cutoffs are always printed on your lab report.

What does a low potassium mean?

Below 3.5 mmol/L is called hypokalaemia. It causes muscle weakness, cramps, fatigue and can trigger arrhythmias. The most common causes are diuretics (water pills), prolonged diarrhoea or vomiting. A low potassium that won't rise with supplementation often points to a magnesium deficiency — so test magnesium too.

What does a high potassium mean?

Above 5.0 mmol/L is called hyperkalaemia. It slows the electrical conduction in the heart. The most common cause is declining kidney function; ACE inhibitors and potassium-sparing diuretics also raise it. An unexpectedly high result is almost always checked for haemolysis (cells breaking in the tube) first and re-tested.

When is an abnormal potassium concerning?

A persistently abnormal value confirmed on two measurements warrants investigating the cause. ECG changes can start to appear from around 6.0 mmol/L, and a potassium of 6.5 mmol/L or higher is a medical emergency. Muscle weakness or palpitations alongside an abnormal potassium call for prompt medical assessment.

How do I read my potassium result?

Always read potassium alongside sodium, creatinine and eGFR — your kidney function largely determines how your body handles potassium. If the result doesn't fit how you feel, a re-test is sensible, especially with an unexpectedly high value.

Related biomarkers

Sodium
Sodium is the main electrolyte outside your cells and, together with water, sets your fluid balance.
Magnesium
Magnesium is a mineral involved in energy production, muscle function, and nerve signalling.
Creatinine
Creatinine is a waste product of your muscles that your kidneys filter from the blood — a measure of kidney function.
eGFR
eGFR is a calculated estimate of how fast your kidneys filter your blood, giving a general picture of kidney function.

Potassium is one of the biomarkers in the Optimize blood test. Book a blood draw at any of 238+ partner labs in the Netherlands, or upload your existing results in the app.

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