Potassium (serum)

The test measures potassium concentration in serum, or blood. Potassium is an electrolyte essential for heart, muscle and nerve function, so even minor abnormalities may matter. It is most often measured in kidney disease, high blood pressure and with medicines affecting potassium, such as diuretics and ACE inhibitors.

Who the test is for

• people taking medicines affecting potassium: diuretics, ACE inhibitors, sartans, spironolactone or potassium preparations;
• people with chronic kidney disease, diabetes or high blood pressure;
• people with vomiting, diarrhoea or dehydration;
• people with muscle weakness and cramps, palpitations or irregular heart rhythm;
• people before surgery and adults having general check-ups.

What potassium is and what the test indicates

Most body potassium is inside cells, with very little in blood. Even small shifts from cells into blood or vice versa therefore noticeably change the result. Potassium is shown as the chemical symbol K, often with a sample designation such as S-K for serum.

We obtain potassium from food, mainly fruit, vegetables, legumes and potatoes. The World Health Organization recommends adults obtain at least approximately 3,5 g daily from food, equivalent to about 90 mmol. Excess potassium is excreted mainly by the kidneys with help from aldosterone, so blood potassium depends primarily on kidney function, hormones and medicines.

Preparation and sample

Venous blood is taken. Fasting is not required unless other tests that day need it. Take medicines as prescribed, and state which medicines and supplements you take at sampling.

Sampling technique matters for potassium. Keep your arm relaxed and do not repeatedly clench and release your fist: muscle activity releases potassium from cells and may falsely raise the result. Haemolysis, red blood cell breakdown in the sample, also falsely raises potassium because these cells contain a lot of it. Potassium cannot be reliably measured in a haemolysed sample, so sampling is repeated.

How results are interpreted

Potassium is expressed in mmol/L. The adult serum reference interval is usually approximately 3,9 to 5,1 mmol/L, with exact limits depending on the laboratory and method and printed on your report. A doctor interprets results alongside symptoms, medicines, creatinine and other electrolytes.

High potassium (hyperkalaemia)

With unexpectedly high potassium, false elevation from haemolysis or fist clenching is excluded first, so testing is often repeated. Truly high potassium most often results from acute or chronic reduced kidney function and medicines: ACE inhibitors, sartans, spironolactone and other potassium-sparing diuretics, non-steroidal anti-inflammatory medicines or potassium preparations. Less common causes include adrenal hormone deficiency, severely uncontrolled diabetes and muscle or other tissue breakdown.

High potassium often causes no symptoms but can cause muscle weakness and heart rhythm disturbances. Potassium around 6 mmol/L or above, particularly with weakness, palpitations or feeling faint, requires urgent medical assessment.

Low potassium (hypokalaemia)

The most common causes are diuretics, vomiting, diarrhoea and excessive laxative use. Potassium is also lost with excess aldosterone, which may cause high blood pressure and is often accompanied by low magnesium. Inadequate dietary intake is rarely the sole cause.

Mildly low potassium may cause no symptoms. More pronounced deficiency causes fatigue, muscle weakness and cramps, constipation and palpitations. Values below 2,5 mmol/L, weakness that makes movement difficult or an irregular heartbeat require emergency care. If the cause is unclear, a doctor may request urine potassium, which shows whether potassium is being lost through the kidneys.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed on the Contact page). Prices for all tests and panels are available in the service price list.

Frequently asked questions

What does mildly high blood potassium mean?

Mildly high potassium without symptoms often results from sampling itself: fist clenching, a prolonged tourniquet or sample haemolysis. A doctor therefore usually requests repeat testing. If elevation is confirmed, kidney function and medicines are checked.

What is the blood potassium abbreviation?

Potassium is shown as K, from the Latin name kalium. A sample letter often precedes it, such as S-K for serum potassium or U-K for urine potassium.

Should I avoid bananas if my potassium is high?

Healthy kidneys readily excrete dietary potassium, so restriction is usually unnecessary. In chronic kidney disease or with medicines retaining potassium, a doctor may advise fewer potassium-rich foods and avoiding potassium salt substitutes. Do not change diet or medicines without medical agreement.