Pseudohyperkalaemia

Pseudohyperkalaemia is a spuriously raised serum potassium caused by potassium release from cells after the blood sample is taken, rather than any true elevation in the patient. It matters because treating it is both unnecessary and potentially harmful: a patient given insulin, calcium and a potassium binder for a laboratory artefact gains only the risk of hypoglycaemia and hypokalaemia.

Recognised causes include haemolysis during difficult venepuncture, prolonged tourniquet time, fist clenching during sampling, delayed sample processing or transport, sample refrigeration, and marked thrombocytosis or leucocytosis, where potassium is released from the very high cell mass during clotting.

The essential clinical judgement is whether the result fits the patient.

• A high potassium in a well patient with entirely normal renal function, no relevant drugs and a completely normal ECG should prompt suspicion before it prompts treatment
• Ask the laboratory whether the sample was haemolysed, which is the single most useful piece of information and is usually flagged automatically
• Review the FBC: a platelet count above roughly 500 × 10⁹/L or a very high white cell count supports pseudohyperkalaemia by this mechanism
• Repeat the sample with good technique: free-flowing venepuncture, minimal or no tourniquet, no fist clenching, prompt transport to the laboratory without refrigeration
• A plasma sample (lithium heparin, which does not clot) rather than a serum sample avoids the clot-related potassium release seen in thrombocytosis, and is the definitive way to resolve the question in patients with high platelet counts
• A venous or arterial blood gas gives a rapid potassium result on a heparinised sample and is a practical immediate cross-check

Importantly, none of this should delay treatment when the patient is unwell, the ECG is abnormal, or the clinical context makes genuine hyperkalaemia plausible. Suspicion of an artefact is not a reason to ignore a dangerous result, it is a reason to confirm it quickly while remaining prepared to treat.

• Do not treat a suspected artefact in a well patient with a normal ECG and no plausible cause; repeat the sample correctly instead
• Where there is genuine doubt, treat as real hyperkalaemia while the repeat is processed, since the consequences of missing true hyperkalaemia far outweigh those of a single unnecessary dose of calcium
• Correct the sampling technique for future draws, and document the tendency in the notes for patients with known thrombocytosis or difficult venous access, so the same investigation is not repeated at every admission
• In patients with myeloproliferative disease and persistent pseudohyperkalaemia, arrange for potassium to be measured on plasma or blood gas samples routinely
• Communicate clearly with the laboratory, since flagging the clinical question often produces a faster and more useful answer than simply resending the sample

Referral: haematology if an unexplained marked thrombocytosis or leucocytosis is newly identified during the workup.