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Potassium Blood Test: Dangerous Levels and What Causes Them

Potassium is essential for normal heart rhythm and muscle function. Both high and low levels can cause life-threatening cardiac arrhythmias, making potassium one of the most critical electrolytes to monitor.

Written and clinically reviewed by Suman Konda, PharmD, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: · How we check our content

Normal potassium
3.5–5.0 mmol/L
Dangerous low
<3.0 mmol/L
Dangerous high
>6.0 mmol/L
Most common drug cause
ACE inhibitors (high K)
Critical Potassium ValuesPotassium >6.5 mmol/L or <2.5 mmol/L is a cardiac emergency requiring urgent ECG and immediate treatment. Do not delay.

Causes of High Potassium (Hyperkalaemia)

CauseMechanism
Renal failureKidneys cannot excrete potassium
ACE inhibitors / ARBs / spironolactoneBlock aldosterone → potassium retention
Acidosis (DKA, metabolic)H⁺ shifts K⁺ out of cells into blood
Haemolysis / haemolysed sampleRed cell contents released (check sample first)
Addison's diseaseLack of aldosterone → K⁺ retention
RhabdomyolysisMassive muscle breakdown releases K⁺

Causes of Low Potassium (Hypokalaemia)

CauseMechanism
Loop / thiazide diureticsIncreased renal potassium excretion (most common)
Vomiting / diarrhoeaGI loss
HypomagnesaemiaRequired for potassium retention (fix Mg first)
Conn's syndrome (hyperaldosteronism)Aldosterone drives K⁺ excretion
Insulin + glucoseShifts K⁺ into cells (used to treat hyperkalaemia)
Check the SampleHaemolysed blood samples falsely elevate potassium: always check if the sample was haemolysed before acting on a high result. Request a repeat if in doubt.
What ECG changes does high potassium cause?
Peaked T waves → widening QRS → sine wave pattern → ventricular fibrillation and cardiac arrest. ECG must be performed urgently in hyperkalaemia.
How is high potassium treated?
Calcium gluconate (cardiac membrane stabilisation: immediate), insulin + glucose (shifts K⁺ into cells, 30 min), calcium resonium or patiromer (gut binding, hours), dialysis for severe renal failure.
What are symptoms of low potassium?
Muscle weakness (may affect breathing at severe levels), cramps, fatigue, constipation, palpitations and arrhythmias, polyuria (blocks ADH action), and hypertension.
When does low potassium need IV treatment?
When K⁺ is <3.0 mmol/L, the patient is symptomatic (weakness, arrhythmia), or cannot take oral supplements. IV potassium must be given slowly (max 40 mmol/hr) on cardiac monitoring.
PalpitationsArrhythmia risk

Frequently asked questions

What is worth asking a doctor about an abnormal Potassium?
Whether it needs repeating, whether a medicine or recent illness explains it, how far outside the range it actually falls, and whether it changes management or simply needs watching. Those four questions cover most of what matters.
Is a slightly abnormal Potassium a reason to worry?
Rarely on its own. Reference intervals are built to contain the middle 95% of a healthy population, so mild deviations are common in well people. The size of the deviation, its direction over time and your symptoms decide whether it means anything.
Can medicines change a Potassium result?
Frequently, and this includes things people do not think of as medicines: supplements, herbal products and over-the-counter painkillers. Bringing a current list to the appointment often explains a result faster than repeating the test.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

  1. Hypokalemia. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK482465
  2. Hyperkalemia. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK470284
  3. Potassium. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK539791

Related reading

Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.

Why a high potassium is sometimes not real

Pseudohyperkalaemia is common enough that it should always be considered before acting on an unexpectedly high result in someone who feels well. Potassium sits mostly inside cells, so anything that damages cells between the vein and the analyser releases it into the sample. Prolonged tourniquet time, clenching the fist repeatedly during the draw, a difficult collection through a narrow needle, delayed transport, and samples chilled or shaken all raise the measured value. A very high platelet or white cell count can do the same. If a raised potassium does not fit the clinical picture, the standard response is to repeat it with careful technique before starting treatment.

Medications that raise potassium

  • ACE inhibitors and ARBs, among the most frequent causes in routine practice.
  • Potassium-sparing diuretics such as spironolactone, eplerenone and amiloride.
  • NSAIDs, which reduce renal perfusion and compound the effect of the above.
  • Trimethoprim, including in co-trimoxazole, which acts on the same renal channel as amiloride.
  • Heparin, and potassium-containing salt substitutes, which people often do not think of as medicines.

Why potassium and magnesium travel together

A low potassium that will not correct despite replacement is frequently a magnesium problem. Magnesium depletion promotes renal potassium wasting, and until the magnesium is replaced the potassium continues to leak. This is a routine consideration in people on long-term diuretics, with alcohol dependence, or with significant diarrhoea, and it is why magnesium is often checked alongside a persistently low potassium rather than after several failed attempts at replacement.

Symptoms and why they are unreliable

Both high and low potassium can produce muscle weakness, cramps and palpitations, and both can be completely silent until the level is dangerous. Because the heart is the organ at risk, an ECG is often more informative than symptoms: peaked T waves, a widening QRS and loss of P waves indicate hyperkalaemia needing immediate treatment, while flattened T waves and prominent U waves suggest hypokalaemia. The absence of ECG changes does not make a markedly abnormal potassium safe to ignore.