- 01A high result in the tube can be normal in the body
Many mildly high results come from the blood draw itself: fist clenching alone raised measured potassium by about 1 to 2 mmol/L in a 1990 New England Journal of Medicine report 3, and a long tourniquet, broken red cells or very high platelets do the same. The first job after a mild high with no symptoms is a clean repeat test; the number and symptoms on that repeat decide whether it is routine, prompt or urgent.
Potassium is one of the few blood results where the size of the number decides how fast you act. A reading of 5.3 mmol/L on a routine panel usually means a repeat test and a look at your medicines; a reading of 6.6 mmol/L means the emergency department today. Potassium sets the electrical charge of every heart muscle cell, and too much of it makes the heartbeat unstable. Some high results are not real at all: they are produced in the tube, not in the body.
The marker this page is about
What counts as a high potassium level
Most US labs put the normal range for serum potassium at about 3.5 to 5.0 mmol/L, with some labs stretching the top to 5.1 or 5.2. For potassium, mmol/L and mEq/L are the same number because potassium carries a single charge 1. The UK Kidney Association grades high potassium in adults as mild at 5.5 to 5.9 mmol/L, moderate at 6.0 to 6.4 mmol/L and severe at 6.5 mmol/L or above 2. Results between the lab's upper limit and 5.5 mmol/L sit in a gray band that labs flag but guidelines do not grade.
Where an example potassium of 5.3 mmol/L fallsmmol/L
| Potassium (mmol/L) | What it is called | What usually happens next |
|---|---|---|
| 5.1 to 5.4 | Borderline high | Repeat test with a clean draw within days; check kidney numbers and medicines |
| 5.5 to 5.9 | Mild hyperkalemia | Repeat within a day or two, medicine review, look for a cause; an ECG is common |
| 6.0 to 6.4 | Moderate hyperkalemia | Same-day medical assessment with an ECG |
| 6.5 and above | Severe hyperkalemia | Emergency treatment and heart monitoring |
Read your exact number on its own page: potassium 5.0, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7 and 6.0. A low result has its own page: low potassium.
Symptoms of high potassium
Most people with a mildly high potassium level feel nothing. Symptoms tend to appear only when potassium is well above the normal range, and they are vague: muscle weakness or heavy legs, tingling or numbness, nausea, tiredness, palpitations, or a slow or irregular pulse. In severe cases weakness can progress to paralysis. Symptoms track the number poorly, so feeling well does not prove a high potassium is safe, and feeling tired does not mean the potassium caused it. Tingling in the hands and feet has many commoner causes. Any high potassium with these symptoms is urgent, whatever the number; the emergency section below lists exactly when to go.
Why high potassium affects the heart
About 98% of the body's potassium sits inside cells, and only about 2% is in the blood that a test measures 1. The ratio between inside and outside sets the resting voltage of nerve and muscle cells, including heart muscle. When potassium outside the cells rises, heart cells become electrically unstable. An ECG (a quick tracing of the heart's electrical activity) first shows tall, pointed T waves, one part of each beat, then slower signals through the heart, and at the extreme dangerous rhythms or cardiac arrest. An ECG is the quick bedside check, but it misses a lot. In a review of hospital patients with significant hyperkalemia, many had none of the classic ECG changes 4, so a normal ECG does not make a high potassium safe.
Why a high potassium result is often false
Pseudohyperkalemia means the potassium is high in the tube but normal in the body. Red cells, platelets and white cells are full of potassium, and anything that breaks them or makes them leak during or after the draw pushes the measured number up. Repeated fist clenching during a blood draw raised measured potassium by about 1 to 2 mmol/L in a 1990 report in the New England Journal of Medicine 3. Other causes include a tourniquet left on too long, a difficult draw that breaks red cells (the lab may report the sample as hemolyzed), a sample left unspun for hours or chilled in transit, and very high platelet or white cell counts, which release potassium as serum clots 1.
- 1Keep your hand relaxedDo not pump or clench your fist while the needle is in.
- 2Ask for a short tourniquet timeThe band should come off once blood starts flowing.
- 3Ask whether a plasma sample is possibleA plasma (green-top, heparin) tube avoids the potassium that platelets release during clotting, which matters most when platelets or white cells are very high.
- 4Get it processed quicklyA draw at the lab itself, or a sample spun soon after, avoids leakage from cells sitting in the tube.
More on getting a clean sample is in how to prepare for a blood test, and why trends beat single readings explains why one reading is only a snapshot.
What causes high potassium
Once a high potassium is confirmed, the causes fall into three groups. The kidneys clear about 90% of the potassium you eat 1, so the first group is reduced kidney clearance: chronic kidney disease, acute kidney injury, dehydration, and low aldosterone, the adrenal hormone that tells the kidneys to excrete potassium, as in Addison's disease. The second group is potassium moving out of cells: high blood sugar with too little insulin in diabetes, acidosis (blood that is more acidic than normal), crushed or broken-down muscle, and some medicines. The third group is too much going in, from potassium supplements, potassium-based salt substitutes or intravenous fluids. Intake alone rarely causes hyperkalemia; it matters when one of the first two groups is already present 1.
| Medicine | How it raises potassium |
|---|---|
| ACE inhibitors (lisinopril, ramipril) and ARBs (losartan, valsartan) | Lower aldosterone, so the kidneys keep more potassium |
| Spironolactone, eplerenone | Block aldosterone's action in the kidney |
| Amiloride, triamterene | Block the kidney sodium channel that drives potassium excretion |
| Trimethoprim (in Bactrim) | Blocks the same kidney channel as amiloride, more so at high doses or with weak kidneys |
| NSAIDs (ibuprofen, naproxen) | Reduce kidney blood flow and the hormone signal that tells the kidneys to excrete potassium |
| Heparin | Suppresses aldosterone production |
| Tacrolimus, ciclosporin | Reduce potassium excretion in the kidney |
| Beta-blockers | Slow the shift of potassium into cells |
| Digoxin (at toxic levels) | Blocks the pump that moves potassium into cells |
Combinations raise the risk most, and spironolactone with an ACE inhibitor is the classic pair. In Ontario, after the 1999 RALES trial showed spironolactone helped heart failure, prescriptions climbed, and hospital admissions for hyperkalemia among older adults with heart failure who were already taking an ACE inhibitor rose from 2.4 per 1,000 in 1994 to 11.0 per 1,000 in 2001, with related deaths rising from 0.3 to 2.0 per 1,000 6. This is why the KDIGO kidney guideline advises checking potassium and creatinine within 2 to 4 weeks of starting or raising the dose of an ACE inhibitor or ARB 5.
The other numbers that explain high potassium
A potassium result reads best beside the rest of the same panel. A raised creatinine or a low eGFR (your estimated kidney filtering rate) points to the kidneys (high creatinine, high BUN, BUN being blood urea nitrogen). A low bicarbonate with a high anion gap (a sum from sodium, chloride and bicarbonate that flags extra acid; work it out with the anion gap calculator) points to acidosis pushing potassium out of cells (low bicarbonate). A high fasting glucose points to an insulin shortfall. Low sodium together with high potassium is the classic pattern of low aldosterone, which is usually measured together with renin, the kidney hormone that drives it. If muscle injury is suspected, a doctor may also check creatine kinase (CK), which rises when muscle breaks down. The metabolic panel guide and electrolytes explained walk through the whole report.
Read these beside potassium
CalculatoreGFR calculatorKidney clearance is one of the most common real causes of high potassium; work out your eGFR from your creatinine.Work it out How to lower high potassium
What a doctor does depends on the number, the symptoms and the cause. For a mild, confirmed high in someone who feels well, the usual order is below.
- Confirming it with a clean repeat test before anything is changed.
- Reviewing medicines and supplements with the prescriber. Do not stop a heart or kidney medicine on your own: ACE inhibitors, ARBs and spironolactone protect the heart and kidneys, and stopping them has its own risks.
- Addressing triggers with the doctor: dehydration, high blood sugar, regular NSAID use, potassium supplements or potassium salt substitutes.
- Treating the cause, such as adjusting a diuretic or managing kidney disease.
- If potassium stays high on a protective medicine, a doctor may add a potassium binder.
Questions worth asking your clinician: can the repeat be a plasma sample, drawn with a relaxed hand? What are my creatinine and eGFR? Could any of my medicines or supplements be raising it?
Two newer oral potassium binders, patiromer (FDA approved 2015) and sodium zirconium cyclosilicate (FDA approved 2018), trap potassium in the gut so it leaves in stool 1. The KDIGO 2024 kidney guideline favors lowering potassium over dropping protective drugs such as ACE inhibitors and ARBs (together called renin-angiotensin system inhibitors, or RASi), a decision made with the prescribing doctor 5.
Hyperkalemia associated with use of RASi can often be managed by measures to reduce the serum potassium levels rather than decreasing the dose or stopping RASi.
In hospital, severe hyperkalemia is treated in steps: intravenous calcium to steady the heart within minutes, insulin with glucose to push potassium back into cells, inhaled albuterol (salbutamol), and then removal of potassium through the kidneys, the gut or dialysis 2. These are emergency treatments given with heart monitoring.
Food and high potassium: when diet matters
With healthy kidneys, food alone rarely causes high potassium 1, so cutting bananas after one borderline result is usually unnecessary. Diet matters when kidney function is reduced or when you take a potassium-raising medicine; then a kidney dietitian can tailor the plan. Salt substitutes are an easy source to miss, because 'lite' and 'no salt' products replace some or all sodium chloride with potassium chloride.
| Higher-potassium food | Lower-potassium swap |
|---|---|
| Potatoes, sweet potatoes | Rice, pasta, or potatoes peeled, cut small and boiled (leaching) |
| Bananas, oranges, melon | Apples, berries, grapes, pineapple |
| Tomatoes, tomato sauce | Peppers, cucumber, a non-tomato sauce |
| Dried fruit, beans, lentils | Fresh fruit; smaller portions of beans |
| Potassium chloride salt substitute | Herbs, lemon, spices |
When high potassium is an emergency
A confirmed high is worth following up even when you feel fine, because the usual causes, kidney function and medicines, are things your doctor will want to track. The kidney function guide and the electrolytes topic cover the wider picture, and lab report flags explained shows what the H next to a result means.
Free, no accountRead your blood testGot a metabolic panel with a high potassium? Read every line of it, including the kidney numbers beside it.Read my results
Citations
- Palmer BF et al. Clinical management of hyperkalemia. Mayo Clin Proc 2021;96(3):744-762
- UK Kidney Association. Clinical Practice Guideline: Treatment of Acute Hyperkalaemia in Adults
- Don BR et al. Pseudohyperkalemia caused by fist clenching during phlebotomy. N Engl J Med 1990;322:1290-1292
- Montague BT et al. Retrospective review of the frequency of ECG changes in hyperkalemia. Clin J Am Soc Nephrol 2008;3(2):324-330
- KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of CKD. Kidney Int 2024;105(4S):S117-S314
- Juurlink DN et al. Rates of hyperkalemia after publication of the Randomized Aldactone Evaluation Study. N Engl J Med 2004;351:543-551
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
