Key electrolyte for heart rhythm and muscle contraction.
What it is
Serum potassium in mmol/L. Haemolysed samples falsely raise potassium: labs usually flag this.
Potassium is the main intracellular cation; only about 2% sits in plasma, which is why small blood changes matter. UK serum potassium is reported in mmol/L. Haemolysis during hard venepuncture falsely raises the result: always read lab comments.
Why it matters
Both high and low potassium can be dangerous, especially with heart or kidney disease or medicines such as ACE inhibitors, diuretics, or spironolactone. Symptoms can include weakness, palpitations, or, at extremes: life-threatening arrhythmias.
This guide is educational only and cannot diagnose you. Discuss results, especially anything outside your laboratory’s reference interval, or that does not match how you feel, with a GP or other appropriately qualified clinician.
Hypokalaemia causes weakness, cramps, and arrhythmias; hyperkalaemia can be silently dangerous, especially with kidney disease or medicines that spare potassium. Do not take potassium tablets or salt substitutes without advice when levels are abnormal.
This guide is educational only and cannot diagnose you. Discuss results, especially anything outside your laboratory’s reference interval, or that does not match how you feel, with a GP or other appropriately qualified clinician.
How it affects the body
Potassium sets resting membrane potential in muscle and cardiac cells. Small concentration changes have outsized electrical effects.
The Na⁺/K⁺-ATPase maintains steep potassium gradients that set resting membrane potential in nerve, skeletal muscle, and heart. Kidneys and aldosterone dominate long-term balance; shifts in and out of cells occur with acid–base change and insulin.
For athletes
Heavy sweat losses plus poor intake can lower potassium; do not megadose potassium supplements without advice. Recheck if the sample was haemolysed after a hard session.
Sweat potassium losses are smaller than sodium but still real in heat. Recheck haemolysed samples after hard sessions. Cardiac symptoms with abnormal potassium are emergency care, not a coaching chat.
