Potassium
Also known as: serum potassium, K+, plasma potassium
The main positively charged ion inside cells; only about 2% is in the blood, but that small fraction sets the electrical excitability of nerves, muscle and above all the heart. Both high and low levels can cause dangerous heart rhythms.
- Also known as
- serum potassium, K+, plasma potassium
- Units
- mmol/L (the same numerically as mEq/L)
- Measured by
- Electrolytes, Kidney function tests, Electrocardiogram
What it is and what it does
The kidney matches potassium excretion to intake under the control of aldosterone, and insulin and adrenaline shift potassium into cells, which is why a rapid change in blood level can occur without any change in total body stores. Hyperkalaemia is most often a combination of reduced kidney excretion and medicines that block aldosterone or the renin-angiotensin system (ACE inhibitors, ARBs, spironolactone, potassium supplements); it flattens the heart's electrical recovery and can cause cardiac arrest, so a very high result is treated as an emergency and checked with an ECG. Hypokalaemia comes from losses through vomiting, diarrhoea or loop and thiazide diuretics and causes weakness, cramps and arrhythmias, particularly in people taking digoxin. Because red cells are full of potassium, a squeezed, delayed or haemolysed sample reads falsely high.
Why it may be higher
- Kidney impairment, acute or chronic
- ACE inhibitors, ARBs, spironolactone and other potassium-sparing diuretics, trimethoprim, NSAIDs, potassium supplements and salt substitutes
- Addison disease (aldosterone deficiency)
- Tissue breakdown (rhabdomyolysis, tumour lysis), acidosis, insulin deficiency in diabetic ketoacidosis
- Spurious: haemolysed or delayed samples, very high platelet or white-cell counts
Why it may be lower
- Vomiting, diarrhoea, laxative misuse
- Loop and thiazide diuretics, corticosteroids, high-dose salbutamol, insulin treatment
- Low magnesium, Conn syndrome (aldosterone excess), Cushing syndrome
- Poor intake, refeeding, alkalosis
A high or low value can be associated with several different situations. The result is interpreted with other results, symptoms, history and clinical findings.
What can affect the result
- Sample handling: fist clenching, tourniquet time, delay before separation, haemolysis
- Recent insulin, salbutamol or intravenous fluids
- Kidney function and current medicines
Medicines that can change it
Each medicine page lists its effect on this marker under "Effects on tests and results", with the source.