Control of Potassium Quiz

See External Balance. Option B is correct: hyperkalaemia is a plasma potassium concentration above the laboratory's upper limit of normal, usually quoted as above 5.0 to 5.5 mmol/L. Each laboratory sets its own reference range (the range found in healthy people), which is why the cut-off varies slightly; SimpleMed and the MSD Manual both use above 5.5 mmol/L. The reason a small rise matters is the resting membrane potential: potassium sits mostly inside cells, and when the blood level rises the membranes become depolarised (less negative inside), so nerves and muscles become more excitable at first. The result is muscle weakness and, most seriously, cardiac arrhythmias (abnormal heart rhythms) that can end in cardiac arrest. Hyperkalaemia is a laboratory diagnosis based on the number, and the ECG is used to judge how urgent it is, not whether it exists. A simple memory aid: hyper means high, so high potassium; the normal range is about 3.5 to 5.0 mmol/L.

• Definition: Plasma potassium above the laboratory's upper limit of normal, usually above 5.0 to 5.5 mmol/L.
• Diagnosis: Made from the blood level alone, with the ECG judging urgency, not whether it exists.
• Danger: Raised potassium can cause cardiac arrhythmias and cardiac arrest.