Potassium (K): heart and muscles

What is potassium (K)

Potassium is the most important electrolyte inside cells. It is essential for maintaining heart rhythm, muscle contractions, and nerve signal transmission. About 98 % of the body's total potassium is inside cells, so the blood test captures only a small but critically important fraction.

We obtain potassium through food — vegetables, fruits, nuts, dairy products, and meat. The kidneys are the primary organ regulating potassium: they excrete excess potassium in urine and retain it when levels are low.

Reference ranges

The standard potassium range in blood is 3.5–5.1 mmol/L. This range applies to adults. Your laboratory may state slightly different limits — follow the reference range provided by your laboratory.

Important: potassium levels can be falsely altered by improper sample collection (haemolysis). If the result is unexpectedly high, your doctor may ask you to repeat the test.

Why potassium may be elevated

Hyperkalaemia (K > 5.1 mmol/L) can be dangerous because it affects heart rhythm. Possible causes:

• Impaired kidney function — the kidneys are unable to excrete potassium efficiently.
• Medications: ACE inhibitors, angiotensin receptor blockers (ARBs), potassium-sparing diuretics, NSAIDs.
• Adrenal insufficiency (Addison's disease).
• Cell breakdown: in severe trauma, burns, or rhabdomyolysis, potassium is released from cells.
• Uncompensated acidosis (acidic blood environment) shifts potassium from cells into the bloodstream.
• Laboratory error (haemolysis in the sample) — a common cause of a falsely elevated result.

Why potassium may be low

Hypokalaemia (K < 3.5 mmol/L) can also affect heart function and muscle strength. Possible causes:

• Diuretics (especially loop and thiazide-type) — the most common medication-related cause.
• Diarrhoea, vomiting — significant potassium loss through the gastrointestinal tract.
• Insufficient dietary potassium intake.
• Insulin therapy — insulin shifts potassium into cells.
• Excess aldosterone (Conn's syndrome) — the kidneys excrete too much potassium.
• Magnesium deficiency can complicate potassium correction.

What to do next

Potassium deviations are evaluated together with sodium, magnesium, creatinine, and ECG results. Even small changes in potassium can be significant for heart function, so your doctor monitors this value closely, especially when you are taking certain medications.

If potassium is low, your doctor may recommend eating more potassium-rich foods (bananas, avocados, potatoes, spinach, beans) or prescribe supplements. If it is high, the first step is to confirm the result is accurate (not haemolysis) and to review medications and kidney function.

How to monitor changes over time

Monitoring potassium is especially important if you take diuretics, antihypertensive medications, or have kidney disease or diabetes. Regular tests help detect trends in time.

On the QbiLabs platform you can view potassium values on a timeline and compare them with reference ranges.

Questions for your doctor

1. Could my potassium level be related to the medications I take? 2. Should magnesium be checked, since it can affect potassium levels? 3. Should I change my diet to adjust my potassium intake? 4. Could my kidney function be contributing to the potassium deviation? 5. Is an ECG needed to check my heart rhythm?

*This is not medical advice. Consult your doctor for any health-related decisions.*