Before a raised potassium is phoned through as critical, the pre-analytical causes deserve a moment. Haemolysis from a difficult draw or a narrow needle releases intracellular potassium. EDTA carried over from a lavender-top tube — from filling tubes in the wrong order — raises potassium and lowers calcium and alkaline phosphatase together. Delayed separation, refrigeration of whole blood and a very high platelet or white cell count also raise potassium. A potassium of 7 with a total calcium of 1.2 mmol/L strongly suggests EDTA contamination, but repeat urgently and get an ECG rather than assume artefact.
- Check the haemolysis index before reporting a raised potassium.
- Suspect EDTA contamination when high potassium comes with very low calcium and low alkaline phosphatase.
- Ask about time to separation and storage temperature when potassium is unexpectedly high.
- In marked thrombocytosis or leucocytosis, suggest a plasma or heparinised whole-blood potassium.
- Phone a critical value with a comment on suspected artefact rather than withholding it.
Why it matters
Spurious hyperkalaemia prompts needless treatment, and repeating a suspect sample is cheaper than dextrose-insulin.
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