Hyperkalaemia (High Potassium) Interpreter: Real or Spurious?

Decide whether a high potassium is real before deciding how urgent it is. The page checks the sample first — haemolysis, EDTA contamination, high platelets or white cells, a delayed or chilled sample, fist clenching — then grades a real result by the 2023 UK Kidney Association guideline, shows where the KDIGO 2020 grading differs, and says what the ECG changes about urgency.

Last updated 21 Sep 2026

Is the high potassium real, and how urgent is it?

Potassium, ECG, sample checks → spurious or real, severity

Potassium (mmol/L)The reported result. Serum normally reads 0.1 to 0.7 mmol/L higher than plasma, because platelets release potassium as the blood clots.

12-lead ECG

Haemolysis index on this sample

Calcium on the same sampleA very high potassium with a very low calcium is the pattern of EDTA contamination. Light contamination can leave the calcium looking normal.

Platelet and white cell counts

Sample type

Time and temperature before separation

The blood draw

A reason for a true high potassium?Potassium-raising drugs include ACE inhibitors, ARBs, mineralocorticoid antagonists, potassium-sparing diuretics, trimethoprim, NSAIDs and heparin.

Thrombocytosis in a serum sample — measure plasma or whole-blood potassium before treatingExample

A 64-year-old with essential thrombocythaemia has a routine serum potassium of 6.3 mmol/L. Platelets 940 ×10⁹/L. ECG normal. Not haemolysed, calcium normal, separated promptly, uneventful draw. Kidney function normal; no potassium-raising drugs.

Two ways to grade a real high potassium

KDIGO calls its grading an expert opinion-based risk classification. UKKA grades by the number and treats ECG changes separately, with IV calcium whenever they are present.
Potassium (mmol/L)	UKKA 2023	KDIGO 2020 — no ECG changes	KDIGO 2020 — ECG changes
5.0 (or your upper limit) to 5.4	Not hyperkalaemia	Mild	Moderate
5.5 to 5.9	Mild	Mild	Moderate
6.0 to 6.4	Moderate	Moderate	Severe
6.5 or more	Severe	Severe	Severe

Is it real? The pre-analytical checks

UKKA 15.3: when pseudohyperkalaemia is suspected, send paired lithium heparin and serum samples from a large vein, with gentle traction and prompt analysis.
Clue	Mechanism	What to do
Haemolysis index raised	Potassium released from lysed red cells	Recollect; report by your laboratory’s haemolysis limits
Very low calcium, very high potassium	Potassium EDTA from a full blood count tube	Recollect; light contamination may leave calcium normal
Platelets above 500 ×10⁹/L, serum sample	Release from platelets during clotting	Plasma or whole-blood potassium
White cells above 50 ×10⁹/L	Leaky white cells; worse in plasma after a pneumatic tube	Whole blood, hand-carried, measured at once
Delayed, overnight or refrigerated	The red cell pump slows and potassium leaks out	Recollect and separate promptly
Fist clenching, long tourniquet	Potassium released from forearm muscle	Recollect with a relaxed hand

Ask whether it is real, but never let the question delay an ECG

A high potassium is one of the commonest results a laboratory telephones, and a large share are not real. Potassium sits inside cells at many times its concentration in plasma, so anything that damages or starves cells between the vein and the analyser pushes the result up: haemolysis, a clotting sample full of platelets, fragile leukaemic white cells, a sample left overnight or in a fridge, a clenched fist, or potassium EDTA carried over from a full blood count tube. None of these makes the patient ill, and treating them with insulin and glucose can do harm.

So the first question is whether it is real — but it must never delay the second. A normal ECG does not prove the potassium is spurious, and changes on the ECG mean the heart is affected whatever the tube looked like. The UK Kidney Association guideline (final version 2023) asks for an urgent 12-lead ECG at 6.0 mmol/L or more and continuous monitoring at 6.5 or more, and a blood-gas potassium gives a true value within minutes. The practical answer to a suspect result of 6.0 or above is therefore both at once: an ECG and a blood-gas potassium now, and a clean recollection.

Two traps. EDTA contamination is taught as a very high potassium beside an unmeasurably low calcium, and gross contamination looks like that; light contamination does not, and a normal calcium does not exclude it. And in chronic lymphocytic leukaemia the usual advice reverses: heparin plasma can read higher than serum, particularly after a pneumatic tube, so whole blood carried by hand is the sample to trust.

For a real result, the guidelines grade it differently. UKKA grades by the number alone, from 5.5 mmol/L. The KDIGO 2020 controversies conference proposed a grading that starts at 5.0 or the laboratory’s upper limit and lets ECG changes raise the grade. The table sets them side by side. See the haemolysis, icterus and lipaemia interpreter for how haemolysis is flagged and the potassium unit converter for unit conversions. This page supports clinical judgement and does not replace it.

Frequently asked questions

What potassium level is an emergency?

UKKA 2023 grades 6.5 mmol/L or more as severe and asks for continuous cardiac monitoring and emergency treatment. Any level with ECG changes of hyperkalaemia is also treated as an emergency, with IV calcium first.

How can I tell if a high potassium is spurious?

Look for a haemolysis flag, an unexplained low calcium (EDTA contamination), a very high platelet or white cell count, a delayed or chilled sample, or a difficult draw with fist clenching. A blood-gas potassium or a paired lithium heparin and serum sample settles it.

Why is serum potassium higher than plasma potassium?

Platelets release potassium as the blood clots. KDIGO puts the normal difference at 0.1 to 0.7 mmol/L, and it grows with a high platelet count.

Does a normal calcium rule out EDTA contamination?

No. Gross contamination drops the calcium dramatically, but light contamination can raise the potassium while the calcium still looks normal, and it is often missed.

How do UKKA and KDIGO grade hyperkalaemia differently?

UKKA: mild 5.5 to 5.9, moderate 6.0 to 6.4, severe 6.5 or more. KDIGO starts at 5.0 or the upper limit of normal, and ECG changes move a result up one grade.

Related calculators

References

UK Kidney Association. Clinical practice guidelines: treatment of acute hyperkalaemia in adults. Alfonzo A, Harrison A, Baines R, Chu A, Mann S, MacRury M. Final version, October 2023 (review October 2026); supersedes the Renal Association 2020 guideline.
Clase CM, Carrero JJ, Ellison DH, et al. Potassium homeostasis and management of dyskalemia in kidney diseases: conclusions from a Kidney Disease: Improving Global Outcomes (KDIGO) Controversies Conference. Kidney Int. 2020;97(1):42–61. Figure 3 (hypokalaemia) and Figure 4 (severity of acute hyperkalaemia).
Ranjitkar P, Greene DN, Baird GS, Hoofnagle AN, Mathias PC. Establishing evidence-based thresholds and laboratory practices to reduce inappropriate treatment of pseudohyperkalemia. Clin Biochem. 2017;50(12):663–669.
Cornes MP, Ford C, Gama R. Spurious hyperkalaemia due to EDTA contamination: common and not always easy to identify. Ann Clin Biochem. 2008;45(Pt 6):601–603.
Don BR, Sebastian A, Cheitlin M, Christiansen M, Schambelan M. Pseudohyperkalemia caused by fist clenching during phlebotomy. N Engl J Med. 1990;322(18):1290–1292.

Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/