High Potassium Levels Explained
What hyperkalaemia means in your blood test, why it happens, and when it's dangerous — written for Australian patients by health data analysts.
What Is Potassium and Why Does It Matter?
Potassium is an essential electrolyte that regulates the electrical activity of your heart, muscles, and nerves. About 98% of your body's potassium sits inside cells, with only 2% in the bloodstream. Because the blood level is so tightly controlled, even small changes can have significant effects on heart rhythm.
Your kidneys are the primary regulator, excreting about 90% of daily potassium intake through urine. The hormone aldosterone fine-tunes this process. When kidney function declines or aldosterone is blocked (by medications or disease), potassium accumulates in the blood — a condition called hyperkalaemia.
Potassium Ranges and Severity
Reference ranges are consistent across Australian pathology labs. Potassium is reported in mmol/L.
Adults
The standard reference range for serum potassium across all Australian pathology labs.
Children
Slightly different range in paediatric populations. Neonates can have higher normal levels (up to 5.9 mmol/L).
Mild hyperkalaemia
Usually asymptomatic. May be a spurious result from difficult blood draw (haemolysis). Recheck recommended.
Moderate hyperkalaemia
Clinical concern. ECG changes may begin. Medications and kidney function must be reviewed urgently.
Severe hyperkalaemia
Medical emergency. High risk of cardiac arrhythmias and cardiac arrest. Requires immediate treatment.
When Is High Potassium Dangerous? ECG Changes
The danger of hyperkalaemia is its effect on the heart. As potassium rises, the heart's electrical conduction system is progressively disrupted.
5.5 – 6.0 mmol/L
Earliest ECG sign. Often the first clue that potassium is clinically elevated. May be subtle.
6.0 – 7.0 mmol/L
Indicates slowing of atrial conduction. The heart’s electrical system is being affected.
7.0 – 8.0 mmol/L
Ventricular conduction is slowing dangerously. Risk of ventricular tachycardia is increasing.
> 8.0 mmol/L
Immediately life-threatening. The heart’s rhythm can degenerate into cardiac arrest at any moment.
Common Causes of High Potassium
Always rule out a spurious (haemolysed) sample first. After that, medications and kidney disease account for the vast majority.
Haemolysed sample (false positive)
The single most common reason for a high potassium result that is NOT real. When blood cells are damaged during collection (difficult venepuncture, tourniquet left on too long, vigorous shaking of tube), potassium leaks from cells into serum. Your lab may flag this as "haemolysed." Always recheck before acting.
Chronic kidney disease (CKD)
The kidneys excrete 90% of daily potassium intake. As kidney function declines (especially eGFR < 30), potassium excretion drops and levels rise. CKD is the most common genuine cause of persistent hyperkalaemia in Australia.
ACE inhibitors / ARBs
Medications like ramipril, perindopril, irbesartan, and candesartan reduce aldosterone, which drives potassium excretion. These are among Australia’s most prescribed medications for blood pressure and heart failure. Risk is higher when combined with CKD.
Spironolactone / Eplerenone
Potassium-sparing diuretics that block aldosterone. Widely used for heart failure, resistant hypertension, and PCOS. Unlike other diuretics (which lower potassium), these raise it. Your GP will monitor potassium closely.
NSAIDs
Ibuprofen, naproxen, diclofenac, and other anti-inflammatories reduce kidney blood flow and decrease potassium excretion. The effect is worse in patients already on ACE inhibitors or with kidney disease. Even over-the-counter NSAIDs carry this risk.
Diabetic ketoacidosis (DKA)
In uncontrolled diabetes, insulin deficiency causes potassium to shift from inside cells to the blood. Serum potassium may be high even though total body potassium is actually depleted. This is an emergency treated in hospital.
Excessive dietary potassium
Unlikely to cause significant hyperkalaemia in people with normal kidney function. However, in CKD patients, high-potassium foods (bananas, oranges, potatoes, tomatoes, salt substitutes containing KCl) can push levels above safe limits.
Addison’s disease
Adrenal insufficiency reduces aldosterone production, impairing potassium excretion. Rare but important. Often presents with fatigue, low blood pressure, skin darkening, and low sodium alongside high potassium.
What Your GP Will Do Next
The approach depends on the level and whether you have symptoms.
1. Confirm the result
The most important first step. Your GP will check whether the sample was haemolysed. If there is any doubt, they will order a repeat blood test with careful venepuncture technique — no fist clenching, minimal tourniquet time, and prompt processing.
2. Review medications
Your GP will check every medication you take, including over-the-counter drugs. ACE inhibitors, ARBs, spironolactone, amiloride, NSAIDs, potassium supplements, and trimethoprim can all raise potassium.
3. Check kidney function
Creatinine, eGFR, and urine ACR will be checked if not already done. Impaired kidney function is the most common genuine cause. If kidney disease is found, your GP may refer to a nephrologist.
4. Order an ECG
If potassium is above 6.0, an ECG is essential to check for cardiac effects. ECG changes (peaked T waves, widened QRS) indicate the heart is at risk and treatment needs to be escalated.
5. Dietary review
For CKD patients with borderline potassium, a dietitian referral can help reduce dietary potassium. In Australia, APD consultations may be covered under Medicare CDM plans for CKD patients.
6. Emergency treatment (if needed)
For potassium above 6.5 or any ECG changes: IV calcium gluconate protects the heart, insulin+glucose shifts potassium into cells, and sodium bicarbonate may be given. These are hospital-level treatments — your GP will send you to ED.
Frequently Asked Questions
Can clenching my fist during the blood draw cause a false high potassium?
Yes. Repeated fist clenching during venepuncture causes local muscle cells to release potassium, and it can raise the result by 0.5–1.0 mmol/L. This is called pseudohyperkalaemia. If your result was mildly elevated and you were clenching, a repeat test is warranted.
Are bananas really that bad for potassium?
Bananas contain about 420 mg of potassium each, which is moderate. For people with normal kidneys, eating bananas is fine. For CKD patients with eGFR below 30, high-potassium foods may need to be limited. A renal dietitian can provide personalised guidance.
Can high potassium cause symptoms I would notice?
Mild hyperkalaemia (5.1–5.9) rarely causes symptoms. Moderate to severe levels may cause muscle weakness, tingling, numbness, nausea, or an irregular heartbeat. The danger is that cardiac arrhythmias can occur without warning symptoms.
I take an ACE inhibitor. Should I stop it because of high potassium?
Never stop a prescribed medication without talking to your GP first. ACE inhibitors and ARBs protect the heart and kidneys, and their benefits usually outweigh the potassium risk. Your GP may adjust the dose or add a potassium-lowering agent.
How quickly can potassium levels change?
Potassium can change rapidly — within hours. A large meal, insulin administration, acid-base changes, or starting/stopping medications can all shift levels quickly. This is why acute hyperkalaemia is treated as an emergency.
What is the difference between serum and plasma potassium?
Serum potassium (from a clotted tube) is typically 0.1–0.5 mmol/L higher than plasma potassium (from a heparinised tube) because platelets release potassium during clotting. Most Australian labs measure serum potassium.
Related Reading
Track Your Electrolytes Over Time
Upload your blood test results and SmarterBlood will chart your potassium, sodium, and kidney function trends automatically — so you can see patterns your GP needs to know about.
This information is based on guidelines from the Royal College of Pathologists of Australasia (RCPA), Kidney Health Australia, and the Australian Resuscitation Council. Reference ranges may vary between pathology providers. SmarterBlood provides educational information only and is not a substitute for professional medical advice.