- 01A surprise high is a question about the tube before it is a question about you
About 98% of potassium sits inside cells, so a small leak in the tube can push a normal 4.5 to a flagged 5.5 or higher. A careful repeat tells the two apart: an artifact reads normal on the repeat, and a true high stays high. Until then the result is treated as real. A level of about 6.0 mmol/L or higher, or palpitations, needs same-day care. Chest pain, fainting or severe weakness needs emergency care.
A high potassium on a routine blood test often comes from the sample itself, so the usual first step is a careful repeat draw within a few days, or sooner. A low potassium most often traces back to diuretics (water pills), vomiting or diarrhea. Normal serum potassium is about 3.5 to 5.0 mmol/L (millimoles per litre) 1. Serum is the clear liquid left after a blood sample clots, and it is what the lab measures. Potassium that is too high or too low can upset the heart's rhythm. A result of about 6.0 mmol/L or higher, a result below about 3.0 mmol/L, or palpitations with any abnormal level needs same-day medical care, even before a repeat. Chest pain, fainting or severe muscle weakness with an abnormal potassium is an emergency: call 911 or your local emergency number.
Normal range and where your number sits
Most US labs report serum potassium against a reference range of about 3.5 to 5.0 mmol/L. The KDIGO (Kidney Disease: Improving Global Outcomes) potassium conference used the same span as the usual normal range 1. The upper edge varies: some labs flag at 5.1, others at 5.3, so a 5.2 can read as normal on one report and high on another. For potassium, mmol/L and mEq/L (milliequivalents per litre) give the same number. Lab units explained and reference ranges explained cover why ranges differ between labs.
Example: a serum potassium of 5.6 mmol/Lmmol/L
| Serum potassium (mmol/L) | How it is usually read | Typical next step |
|---|---|---|
| Below 3.0 | Clearly low; higher risk of heart rhythm problems and muscle weakness | Same-day medical care |
| 3.0 to 3.4 | Mildly low | Clinician reviews causes (diuretics, gut losses, magnesium); ask your clinician when to recheck |
| 3.5 to 5.0 | Usual reference range | No action from this number alone |
| 5.1 to 5.5 | Borderline or mildly high, depending on the lab's upper limit | Prompt careful repeat plus kidney check |
| 5.6 to 5.9 | High if confirmed | Prompt repeat; clinician reviews kidneys and medicines |
| 6.0 or above | Potentially dangerous for heart rhythm if real | Same-day medical care |
These bands are a common clinical convention built around the KDIGO normal range. Guidelines and hospitals draw the exact edges a little differently. When a result is high, clinicians also weigh symptoms, kidney function and an ECG (heart tracing) 1.
Why most potassium sits inside cells, and why that matters
An electrolyte result is a concentration, the amount of a mineral per litre of blood water, so it changes when water moves as well as when the mineral does.
About 98% of the body's potassium is inside cells, and only about 2% circulates in the blood 1. So the serum number is a small outside reading of a large inside store. As a result:
- A tiny leak from cells into the tube can push the reading up even when the body is fine.
- Insulin and the blood's acid level move potassium into or out of cells. That changes the reading without changing the body's total 1.
Over time, the kidneys set the level. They remove most of each day's potassium intake 1.
Why the sample can read falsely high
A falsely high potassium, called pseudohyperkalemia, comes from potassium that leaked into the tube after the blood left the body. In a 1990 study in the New England Journal of Medicine, repeated fist clenching during a blood draw raised measured potassium by about 1 mmol/L or more 25. That is enough to turn a normal 4.5 into a flagged 5.5 or higher.
- Fist clenching or pumping during the draw, or a tourniquet left on a long time 25.
- Hemolysis: red blood cells bursting in the tube, often after a difficult draw. Many lab reports add a hemolysis flag or comment 5.
- Delayed processing, where the sample sits for hours before the serum is separated. The cells run out of fuel and potassium leaks out 5.
- Cold storage or transport, which slows the pumps that keep potassium inside cells 5.
- Very high platelet or white-cell counts, which release potassium as the blood clots 5. Your complete blood count from the same day shows whether this applies.
| Sample artifact (pseudohyperkalemia) | True high potassium (hyperkalemia) | |
|---|---|---|
| Where the potassium came from | Leaked out of cells in the tube | Raised in the bloodstream itself |
| Typical clues | Hemolysis flag, hard draw, delayed or chilled sample, very high platelets | Reduced eGFR, potassium-raising medicines, symptoms, ECG changes |
| Repeat result | Normal on a careful repeat | Still high on a careful repeat |
| Risk to the heart | None from the artifact itself, but a true high cannot be ruled out until a careful repeat | Can cause dangerous rhythm changes |
Causes of a true high potassium
Reduced kidney function
Healthy kidneys get rid of most of the potassium you eat each day, so a true high potassium most often appears when kidney function falls 1. The first numbers a clinician checks are creatinine and the eGFR (estimated glomerular filtration rate, a measure of how much blood the kidneys filter each minute) on the same panel. See kidney function explained and chronic kidney disease for how they are read. If you have a creatinine result, the calculator below turns it into an eGFR.
Estimate your kidney filtration (eGFR)
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Medicines
KDIGO lists several common drug groups that raise potassium. The risk rises sharply as kidney function falls 1:
- ACE inhibitors (for example lisinopril, ramipril) and ARBs, or angiotensin receptor blockers (for example losartan, valsartan), used for blood pressure, heart failure and kidney protection.
- Mineralocorticoid antagonists, a type of potassium-sparing diuretic: spironolactone and eplerenone.
- NSAIDs (non-steroidal anti-inflammatory drugs) such as ibuprofen and naproxen.
- Trimethoprim, an antibiotic often combined with sulfamethoxazole.
- Heparin, a blood thinner.
Many of these medicines protect the heart and kidneys, and a mild rise in potassium is often managed without stopping them. Your clinician may review the ones that raise potassium. Never stop or change one yourself because of a single lab result.
Shifts out of cells
Insulin moves potassium into cells. A lack of insulin with very high blood sugar can therefore raise serum potassium; see blood sugar explained and fasting glucose. Acid buildup in the blood pushes potassium out of cells. That is one reason a clinician reads potassium next to bicarbonate, the panel's acid-base marker 1.
Potassium-based salt substitutes
What lowers potassium
Low potassium (hypokalemia) usually comes from losses through the kidneys or the gut 1:
- Loop diuretics (such as furosemide) and thiazide diuretics (such as hydrochlorothiazide or chlorthalidone).
- Vomiting.
- Diarrhea.
- Regular laxative use.
Low potassium raises the risk of irregular heart rhythms, especially in people with heart disease or those taking digoxin, a heart-rhythm medicine 1.
When low potassium will not come up: check magnesium
Magnesium deficiency is frequently associated with hypokalemia.Huang CL, Kuo E. J Am Soc Nephrol. 2007 3
Huang and Kuo describe the mechanism. Low magnesium inside kidney cells takes the brake off ROMK channels, the gates that let potassium leak into urine, so the kidneys keep losing potassium 3. Potassium often stays low until magnesium is corrected. Ask your clinician whether a serum magnesium test belongs on the next draw. The magnesium page covers the mineral itself.
Both ends of the range carry risk
In large observational datasets reviewed by KDIGO, the link between potassium and death is U-shaped. Risk is lowest in the middle of the normal range and climbs toward both ends, most clearly in people with chronic kidney disease, heart failure or diabetes 1. Observational data show an association. They do not prove the potassium level caused the deaths. In practice, it means a result near either edge is worth a recheck and a conversation, not alarm.
Reading potassium with its neighbours
A potassium number is rarely read alone. These markers, on the same panel or a nearby one, change what it means:
| Marker | What it tells you about a potassium result | When it matters most |
|---|---|---|
| eGFR and creatinine | Whether the kidneys can clear potassium | A high potassium, or starting a potassium-raising medicine |
| Magnesium | Whether the kidneys are being pushed to lose potassium | A low potassium that does not respond to treatment |
| Bicarbonate | Acid-base state, which shifts potassium in or out of cells | Potassium that is off along with bicarbonate |
| Glucose | Insulin and sugar levels, which move potassium into cells | Very high blood sugar or diabetes treatment |
| Sodium | Water balance, read next to potassium on the same panel | Diuretic use, where both can fall together |
How it is tested and when it is rechecked
- Serum potassium is part of the basic metabolic panel (BMP) and the comprehensive metabolic panel (CMP), so it is rarely ordered on its own. Electrolytes explained walks through the rest of the panel.
- You do not need to fast for potassium itself. Other tests on the same panel, such as glucose, may need fasting.
- Serum potassium reads a little higher than plasma potassium because clotting releases potassium from platelets, on average by about 0.36 mmol/L 5. A lab's range applies to its own sample type 1.
- People starting or changing an ACE inhibitor, ARB, spironolactone or eplerenone usually have potassium and creatinine rechecked within weeks. People with chronic kidney disease have it checked on a schedule their clinician sets 1.
One flagged value matters less than the pattern across draws. Trends beat single readings explains why a careful repeat is the standard response to a surprise result.
Diet and your potassium result
US Adequate Intakes for potassium are 3,400 mg a day for men and 2,600 mg a day for women 4. With healthy kidneys, blood potassium stays in range across a wide span of intakes, so a serum potassium result is not a measure of your diet 4. In people with healthy kidneys, an abnormal result usually does not mean you ate too much or too little potassium-rich food. Diet matters most when kidney function is reduced, and that is a conversation for your clinician. More on the whole group is under electrolytes and kidney.
Citations
- Clase CM, Carrero JJ, Ellison DH, et al. Potassium homeostasis and management of dyskalemia in kidney diseases: conclusions from a KDIGO Controversies Conference. Kidney Int. 2020;97(1):42-61.
- 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.
- Huang CL, Kuo E. Mechanism of hypokalemia in magnesium deficiency. J Am Soc Nephrol. 2007;18(10):2649-2652.
- National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes for Sodium and Potassium. Washington, DC: The National Academies Press; 2019.
- Asirvatham JR, Moses V, Bjornson L. Errors in potassium measurement: a laboratory perspective for the clinician. N Am J Med Sci. 2013;5(4):255-259.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.