Hyperkalemia (high potassium)

Hyperkalemia is a raised level of potassium in the blood. Potassium is one of the body's key electrolytes and is tightly controlled, because it is critical to the electrical signalling that runs the heart and muscles. The kidneys are the main route for removing excess potassium, so the commonest cause of a genuinely high level is reduced kidney function — either chronic kidney disease or acute kidney injury. Many widely used medications also raise potassium, including some blood-pressure drugs (ACE inhibitors, ARBs), potassium-sparing diuretics and potassium supplements. It can also rise when potassium shifts out of cells, as in severe tissue damage, or in acidosis such as diabetic ketoacidosis, and rarely from adrenal problems (low aldosterone, as in Addison's disease). Importantly, a common cause of an apparently high result is not real: if the blood sample is difficult to draw or the red cells break down in the tube, potassium leaks out and gives a falsely high reading (pseudohyperkalemia), so a surprising result is often rechecked. A mildly raised potassium often causes no symptoms, but a high level can cause muscle weakness and, most seriously, dangerous changes to the heart's rhythm. Severe hyperkalemia is a medical emergency.

• Potassium — the defining test; the height and speed of the rise guide urgency.
• Kidney function (creatinine, eGFR) — the commonest underlying driver.
• Bicarbonate — a metabolic acidosis can accompany or contribute to it.
• Glucose — high in diabetic ketoacidosis, a cause of potassium shifts.
• A repeat/confirmatory sample — to exclude a false high from a difficult draw (pseudohyperkalemia).

• Often no symptoms, especially if mild
• Muscle weakness or fatigue
• Numbness or tingling
• Palpitations or an irregular heartbeat
• In severe cases, a dangerously abnormal heart rhythm and collapse
• Symptoms of the underlying cause (e.g. reduced urine output in kidney failure)