K

Potassium: normal 3.5–5.0 mmol/L; causes and dangers of hypo- and hyperkalaemia, ECG effects and false results.

Potassium (K⁺) is the main positively charged ion inside cells: about 98 % of the body's potassium is intracellular, and only 2 % is in the blood. The difference between potassium inside and outside cells sets the resting electrical potential of nerve and muscle cells, including the heart. Even small deviations of the blood level, too low (hypokalaemia) or too high (hyperkalaemia), can cause muscle weakness and dangerous heart rhythm disturbances.

ElectrolytesHeart rhythmKidney function

Normal serum level3.5–5.0 mmol/L

Hypokalaemia< 3.5 mmol/L

Hyperkalaemia> 5.0–5.5 mmol/L

Critical< 2.5 or ≥ 6.5 mmol/L

Figure 1. Serum potassium zones. Commonly used grading of hyperkalaemia: mild 5.5–5.9, moderate 6.0–6.4, severe ≥ 6.5 mmol/L.

How the body controls potassium

We eat about 50–100 mmol of potassium a day, more than the whole amount in the blood. After a meal, insulin and adrenaline quickly shift potassium into cells. Over the following hours the kidneys excrete the excess under the control of aldosterone. Acid-base balance also matters: acidosis moves potassium out of cells, and alkalosis moves it in.

Low potassium — hypokalaemia

Losses through the kidneys: thiazide and loop diuretics (the most common cause), primary aldosteronism, magnesium deficiency, some kidney tubular disorders
Losses through the gut: vomiting, diarrhoea, laxative misuse
Shift into cells: insulin treatment, salbutamol, alkalosis, refeeding after starvation
Poor intake (rarely the only cause)

Symptoms include weakness, muscle cramps, constipation, palpitations and, in severe cases, paralysis. On the ECG, flattened T waves, ST depression and prominent U waves appear. Hypokalaemia increases the risk of arrhythmias, especially with heart disease or digoxin.

High potassium — hyperkalaemia

Reduced excretion: acute or chronic kidney disease, medicines (ACE inhibitors, angiotensin receptor blockers, spironolactone, eplerenone, potassium-sparing diuretics, NSAIDs, trimethoprim), adrenal insufficiency
Shift out of cells: acidosis, insulin deficiency (diabetic ketoacidosis), tissue breakdown (rhabdomyolysis, burns, tumour lysis)
Pseudohyperkalaemia — false result from haemolysis of the sample, a tight tourniquet, fist clenching, delayed processing or very high platelet or white cell counts. A surprising value without explanation should be repeated promptly.

Hyperkalaemia is often silent until it causes muscle weakness or arrhythmia. ECG changes range from peaked T waves to a widened QRS, slow rhythms and cardiac arrest. ECG changes do not reliably track the potassium level.

Seek emergency care

if potassium is 6.5 mmol/L or higher, or below 2.5 mmol/L, or if an abnormal level comes with palpitations, muscle weakness, fainting or chest discomfort.

Planned evaluation

Mild abnormalities should be rechecked, and medicines, kidney function and magnesium reviewed. Do not start potassium supplements or salt substitutes containing potassium without medical advice, especially with kidney disease or blood-pressure medicines.

References

Clase CM, Carrero JJ, Ellison DH, et al. Potassium homeostasis and management of dyskalemia in kidney diseases: conclusions from a Kidney Disease: Improving Global Outcomes (KDIGO) Controversies Conference. Kidney Int. 2020;97(1):42–61. doi:10.1016/j.kint.2019.09.018
Rautaharju PM, Surawicz B, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part IV: the ST segment, T and U waves, and the QT interval. Circulation. 2009;119(10):e241–e250. doi:10.1161/CIRCULATIONAHA.108.191096

This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

Published on 6 July 2019

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