Potassium — Serum Potassium (K⁺)

What is potassium?

Potassium (chemical symbol K⁺) is an electrolyte — a mineral that, when dissolved in body fluids, carries an electrical charge and enables electrical signalling between cells. It is the most abundant positively charged ion inside cells (intracellular cation), with 98% of the body's potassium held within cells and only a small fraction circulating in the blood.

The concentration difference between potassium inside and outside cells creates the resting membrane potential — the electrical charge across cell membranes that underlies every heartbeat, every nerve impulse, and every muscle contraction. Even small changes in serum potassium disrupt this gradient and have profound effects on cardiac and muscle function. This is why the normal range for potassium is very narrow (3.5–5.0 mmol/L) compared with most other blood tests.

Potassium is regulated primarily by the kidneys, which adjust urinary potassium excretion in response to signals from the hormone aldosterone (produced by the adrenal glands). When aldosterone is high, the kidneys excrete more potassium and retain sodium. When aldosterone is low (as in Addison's disease), potassium accumulates. Many common medications — particularly diuretics and ACE inhibitors — have significant effects on potassium balance, making monitoring essential in patients taking these drugs.

Normal potassium range

The normal adult serum potassium range is:

• Normal: 3.5–5.0 mmol/L
• Mild hypokalaemia: 3.0–3.4 mmol/L — symptoms possible; oral supplementation often indicated
• Moderate hypokalaemia: 2.5–2.9 mmol/L — significant cardiac risk; IV replacement may be needed
• Severe hypokalaemia: <2.5 mmol/L — medical emergency
• Mild hyperkalaemia: 5.1–5.9 mmol/L — investigate and monitor; check for haemolysis
• Severe hyperkalaemia: >6.0 mmol/L — cardiac emergency; ECG changes may occur

What a high potassium may indicate

Hyperkalaemia (high potassium) can cause serious cardiac arrhythmias and warrants prompt investigation. Common causes include:

• Chronic kidney disease — the most common true cause; impaired renal excretion allows potassium to accumulate; potassium rises progressively as kidney function worsens
• ACE inhibitors, ARBs, and spironolactone — these blood pressure medications reduce aldosterone effect, retaining potassium; the combination of these drugs is a particularly high-risk scenario
• Addison's disease — low aldosterone means the kidneys retain potassium and excrete sodium; typically presents alongside hyponatraemia (low sodium)
• Metabolic acidosis — in acidosis, hydrogen ions move into cells and potassium moves out into the blood (transcellular shift); common in diabetic ketoacidosis
• Haemolysis in the blood tube (pseudohyperkalaemia) — red blood cells rupture during or after collection, releasing intracellular potassium into the sample; a very common false-high result that should always be suspected and the test repeated

What a low potassium may indicate

Hypokalaemia (low potassium) impairs muscle and cardiac function and should be investigated and corrected. Common causes include:

• Diuretic medication — furosemide (loop diuretics) and thiazide diuretics are the most common cause; they increase urinary potassium excretion as a side effect; patients on these medications need regular monitoring
• Vomiting and diarrhoea — direct loss of potassium via the gut; persistent vomiting also causes metabolic alkalosis, which drives potassium into cells, further lowering serum levels
• Conn's syndrome (primary hyperaldosteronism) — an aldosterone-secreting adrenal adenoma; classically presents with resistant hypertension and hypokalaemia
• Poor dietary intake — unusual as a sole cause but can contribute in elderly patients, eating disorders, or severe malnutrition
• Insulin and beta-agonist therapy — both cause potassium to shift into cells; used acutely to treat dangerous hyperkalaemia but can cause rebound hypokalaemia

Get your potassium tested at The Vesey

Potassium is included in the following panels at The Vesey Private Hospital, Sutton Coldfield. It is always reported alongside sodium, chloride, and bicarbonate for a complete electrolyte assessment:

• Electrolyte Profile — potassium, sodium, chloride, bicarbonate
• Kidney Health Panel — electrolytes plus eGFR, creatinine, and urea
• Lifestyle Screen — comprehensive multi-system panel including electrolytes
• Advanced Health Panels — full metabolic and organ-function assessment

No fasting required. Results reviewed by a clinician before release.

Frequently asked questions

Further reading: Potassium — Lab Tests Online UK · Independent patient information from the British Society for Clinical Biochemistry.