Serum Potassium
Also known as: K+, Serum K, Hypokalaemia/Hyperkalaemia test
Potassium is the most important intracellular electrolyte and plays a critical role in heart rhythm and muscle function. Even small deviations outside the reference range can cause life-threatening cardiac arrhythmias.
Venous blood (serum separator tube, lithium heparin)
1–2 hours
Not required
Metabolic Panel
Reference Ranges
| Measurement | Reference Range | Unit |
|---|---|---|
| Normal | 3.5 – 5.1 | mmol/L |
| Mild hypokalaemia | 3 – 3.5 | mmol/L |
| Severe hypokalaemia | ≤ 3 | mmol/L |
| Mild hyperkalaemia | 5.1 – 6 | mmol/L |
| Severe hyperkalaemia | ≥ 6.5 | mmol/L |
Reference ranges are typical adult values. Your laboratory may use slightly different intervals — always use your lab's reported reference.
What This Test Measures
Potassium maintains the electrical gradient across cell membranes that drives nerve conduction and muscle contraction — including the cardiac muscle. Ninety-eight percent of body potassium is intracellular; serum levels reflect a small, tightly regulated pool. Acid-base status dramatically affects distribution: acidosis drives potassium out of cells (raising serum levels); alkalosis drives it in (lowering serum levels).
When Is This Test Ordered?
- Monitoring diuretic therapy (loop and thiazide diuretics cause potassium loss)
- Hypertension management
- Kidney disease monitoring
- ACE inhibitor / ARB therapy (these can raise potassium in kidney disease)
- Suspected eating disorders, vomiting, or laxative abuse
- Any significant illness, cardiac, or surgical admission
When Results Are High
Hyperkalaemia above 6.5 mmol/L is a cardiac emergency — peaked T waves, then widening QRS, then ventricular fibrillation. Causes: kidney failure (most common), potassium-sparing diuretics, ACE inhibitors in CKD, Addison's disease. Pseudohyperkalaemia from haemolysis is common (potassium leaks from ruptured RBCs in the tube).
When Results Are Low
Hypokalaemia below 3.0 mmol/L causes muscle weakness, cramps, constipation, and arrhythmias. Causes: diuretics, vomiting, diarrhoea, hyperaldosteronism, low dietary intake. Severe hypokalaemia (below 2.5 mmol/L) can cause paralysis and life-threatening arrhythmias.
Why pseudohyperkalaemia is the most common 'high potassium' result
When blood cells rupture in the collection tube (haemolysis), they release their intracellular potassium — values of 6.5–9 mmol/L with a visually pink or red serum are almost always artefact. If the patient is clinically well with no ECG changes, the correct response is a careful recollect, not emergency treatment.
True hyperkalaemia requires ECG correlation. A potassium of 6.8 mmol/L with a completely normal ECG is almost certainly pseudohyperkalaemia.
Factors That Can Affect Your Result
- Haemolysis — ruptured RBCs release intracellular potassium; re-collect if result seems inconsistent
- Fist clenching during phlebotomy raises potassium by 0.5–1.5 mmol/L
- Prolonged tourniquet time also raises potassium
- Sample temperature — storage at room temperature allows cells to leak potassium
- Insulin drives potassium into cells, lowering serum levels
Tracking This Test Over Time
If you take a loop or thiazide diuretic, your potassium should be checked every 3–6 months. A trend from 3.8 to 3.3 to 3.0 mmol/L over a year signals progressive depletion — act on the trend rather than waiting for a value below 3.5.
Track your Potassium in LoomsRelated Tests
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Sodium is the primary electrolyte that determines blood osmolality and fluid distribution. Both low and high sodium levels can cause severe neurological symptoms and require careful correction.
Magnesium
Magnesium is involved in over 300 enzymatic reactions including ATP production, muscle contraction, and nerve signalling. Deficiency is extremely common but often missed because serum magnesium can remain normal while intracellular stores are depleted.
Creatinine / eGFR
Creatinine is a waste product filtered by the kidneys; eGFR (estimated glomerular filtration rate) converts it into a percentage of normal kidney function. Together they are the primary tools for detecting and staging chronic kidney disease.
Cortisol
Cortisol is the body's primary stress hormone — essential for blood sugar regulation, immune response, and blood pressure. Testing requires strict attention to timing because cortisol follows a powerful circadian rhythm.
Medical disclaimer
This page is for educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories, and results must be interpreted in the context of your symptoms, history, and other tests. Always discuss your results with your healthcare provider. Content reviewed by the Looms medical team, 2026-06-01.