Potassium test explained (K)

Potassium is the main electrolyte inside the cells: most of the body's potassium sits within cells, and only a small but tightly regulated share is in the blood. That amount determines how nerve impulses are passed on and how muscles contract, the heart included — which is why even small deviations can change heart rhythm. Potassium balance is maintained by the kidneys and by the adrenal hormone aldosterone, and it is strongly affected by medicines: diuretics and some blood pressure medicines (ACE inhibitors, angiotensin receptor blockers, potassium-sparing diuretics, beta blockers). Potassium is tested as part of an electrolyte or kidney panel, often together with sodium, creatinine and acid–base status. It is worth knowing that haemolysis in the sample (blood cells breaking down as the blood is taken) raises potassium artificially — an unexpectedly high result is usually repeated by the laboratory or the doctor.

• When kidney disease is present or suspected
• When assessing heart rhythm disturbances and blood pressure problems
• For monitoring while taking diuretics or blood pressure medicines that change potassium
• During prolonged diarrhoea or vomiting, and in dehydration
• When an acid–base imbalance (acidosis, alkalosis) or an adrenal disease is suspected
• As part of an electrolyte panel, as a check-up or before surgery

High potassium (hyperkalaemia) is most often linked to kidney failure and to medicines — ACE inhibitors, angiotensin receptor blockers, potassium-sparing diuretics, beta blockers. Other causes are Addison's disease, acidosis, crushed tissue, a blood transfusion, and too much potassium from food or supplements. Haemolysis in the sample raises potassium artificially, so an unexpected result is repeated.

• Kidney failure
• Medicines: ACE inhibitors, angiotensin receptor blockers, potassium-sparing diuretics, beta blockers
• Addison's disease — adrenal insufficiency
• Acidosis
• Crushed tissue, severe injury
• A blood transfusion
• Too much potassium from food or supplements
• Haemolysis in the sample — an artificial rise

Symptoms: often there are none. Weakness and tiredness, heart rhythm disturbances and nausea can occur.

What to discuss with a doctor after a high result

• A repeat test — whether the result was down to haemolysis
• Creatinine and GFR — kidney function
• The medicines and potassium-containing supplements you take
• An ECG, if potassium is high or there are rhythm disturbances — the doctor decides

Low potassium (hypokalaemia) most often arises through loss — with diarrhoea, with vomiting, while taking diuretics — or from getting too little of it in food. Other causes are Cushing's syndrome, hyperaldosteronism and renal artery stenosis. Low potassium can cause muscle weakness, cramps and heart rhythm disturbances.

• Diarrhoea, vomiting
• Diuretics
• Too little potassium in food
• Cushing's syndrome
• Hyperaldosteronism
• Renal artery stenosis

Symptoms: muscle weakness and cramps, tiredness, constipation, heart rhythm disturbances.

What to discuss with a doctor after a low result

• Whether there was diarrhoea or vomiting, and whether you take diuretics
• Magnesium — a lack of it gets in the way of restoring potassium
• Blood pressure and adrenal hormones, if the cause is unclear
• How and how much potassium to replace — the doctor decides, because an excess is dangerous too

At the Medicina practica laboratory a potassium test costs from €7 (September 2026 pricing). Potassium is not in the free diagnostics programme of the National Health Insurance Fund (VLK), which covers lipids, glucose, and heart and cancer markers — if a doctor orders the test on indications, the cost depends on the particular institution and on insurance cover.