Potassium (K+, K, Serum potassium, Plasma potassium, Potassium blood test, Kalium)
Potassium is one of the body's essential electrolytes — minerals that carry an electrical charge and make nerve signals, muscle contractions, and every heartbeat possible. Nearly all of it, about 98%, is stored inside your cells; the bloodstream carries only a sliver, held within a remarkably narrow window. The potassium blood test measures that sliver, usually as part of a basic or comprehensive metabolic panel.
Your kidneys do most of the work of keeping blood potassium steady, fine-tuning how much leaves in urine hour by hour, with help from hormones like aldosterone and insulin. That is why potassium results are read almost inseparably from kidney function, and why so many common medicines — blood pressure drugs, water pills, even ibuprofen — show up in the story when a result drifts out of range.
Potassium also has a quirk worth knowing before you worry about a number: it is the lab value most often distorted by the blood draw itself. Because red blood cells are packed with potassium, cells that break in the needle or the tube spill their contents and inflate the result. Labs flag these samples as "hemolyzed," and an unexpectedly high potassium in someone who feels well very often turns out to be exactly that.
Reference ranges
Adults
Most US labs use an adult serum potassium reference range of about 3.5 to 5.0 mEq/L (some extend the upper limit to 5.1 or 5.2), and the range is the same for men and women. US references conventionally grade hypokalemia as mild (3.0 to 3.4), moderate (2.5 to 2.9), and severe (below 2.5 mEq/L), and most US hospitals treat results below about 2.5 or at or above about 6.5 as critical values — exact cutoffs vary slightly by lab, and this page follows the most widely used ones. That convention is why the low side of this scale steps directly from a same-day concern into emergency territory: below 2.5, there is no established in-between tier.
| Range (mEq/L) | Classification |
|---|---|
| 1.5–2 | Profoundly low (life-threatening hypokalemia) |
| 2–2.5 | Critically low (severe hypokalemia) |
| 2.5–3 | Low (moderate hypokalemia) |
| 3–3.5 | Slightly low (mild hypokalemia) |
| 3.5–5 | Normal |
| 5–5.5 | Borderline high |
| 5.5–6 | High (hyperkalemia) |
| 6–6.5 | Very high (marked hyperkalemia) |
| 6.5–8 | Critically high (severe hyperkalemia) |
How to read your result
A low result usually means potassium is being lost — through urine (most often diuretics), through the gut (vomiting, diarrhea, laxatives) — or has temporarily shifted from the blood into cells, as insulin and albuterol can cause. A high result usually means the kidneys are not excreting enough (kidney disease, or medicines like ACE inhibitors and spironolactone), potassium has shifted out of cells (tissue injury, severe acidosis), or the sample was hemolyzed and the number is not real.
Three things determine how much a result matters: how far it sits from the roughly 3.5 to 5.0 mEq/L range, how fast it got there, and the state of your heart and kidneys. A slightly odd value in a healthy person is usually a repeat-and-review situation. The widely used critical thresholds — below about 2.5 or above about 6.0 to 6.5 mEq/L — are the territory where same-day or emergency care is warranted, because those levels can disturb the heart's rhythm.
What can shift this value
- Diuretics: thiazides and loop diuretics (hydrochlorothiazide, furosemide) lower potassium; potassium-sparing ones (spironolactone, amiloride) raise it
- Blood pressure medicines: ACE inhibitors and ARBs raise potassium modestly
- Sample handling: hemolysis, fist clenching, a tight tourniquet, or delayed processing falsely raises the result
- Kidney function: as eGFR declines, potassium tends to climb
- Vomiting, diarrhea, and laxative overuse drain potassium
- Insulin and albuterol shift potassium into cells, lowering the blood level temporarily
- Potassium-based salt substitutes and supplements raise it, sometimes sharply in kidney disease
- Magnesium: low magnesium makes low potassium stubbornly hard to correct
- Heavy sweating and endurance exercise lower it, while intense exercise right before a draw can transiently raise it
- NSAIDs (ibuprofen, naproxen) nudge potassium upward, especially alongside kidney disease or ACE inhibitors
When it's tested
Potassium is part of the basic metabolic panel (BMP), the comprehensive metabolic panel (CMP), electrolyte panels, and renal function panels — so it is checked at most physicals, ER visits, and hospital admissions. It is monitored on a schedule for people taking diuretics, ACE inhibitors, ARBs, or spironolactone, and for anyone with kidney disease, heart failure, or an arrhythmia workup. It is not part of a CBC or a lipid panel.
US reports use mEq/L and most other countries use mmol/L, but for potassium the two are numerically identical — 4.2 mEq/L equals 4.2 mmol/L — so no conversion is ever needed.
Frequently asked questions
Is your value a normal potassium level?
For adults, most US labs consider roughly 3.5 to 5.0 mEq/L normal for blood potassium, and some accept up to 5.1 or 5.2. Compare your number to the range printed on your own report, and talk with your clinician about anything outside it — how urgent that conversation is depends on how far outside the range you are and how quickly the level changed.
Can a hemolyzed blood sample make my potassium look falsely high?
Yes, and it happens often. Red blood cells hold enormously more potassium than the liquid part of blood, so if cells rupture during the draw — from a small needle, a tight tourniquet, fist pumping, or rough handling of the tube — potassium spills out and inflates the result. Labs usually flag these samples as "hemolyzed." It is the single most common reason for an unexpectedly high potassium in someone who feels well, and a carefully repeated draw settles the question.
What potassium level is dangerously high or low?
Most US hospitals treat potassium below about 2.5 mEq/L or above about 6.5 mEq/L as critical (panic) values, and many also flag anything at or above 6.0. But danger is not just the number: how fast the level changed, whether your heart or kidneys are already strained, and medicines like digoxin all shift the picture. That is why an identical number can be routine for one person and an emergency for another — your clinician interprets it in context.
How can I fix a low potassium level — can I just eat bananas?
Food helps with mild dips, but it has limits: a medium banana holds about 420 mg of potassium — roughly 10 mEq — while treatment for a genuinely low level often requires far more, in prescription form, along with fixing whatever is causing the loss. Beans, potatoes, leafy greens, citrus, and yogurt are all richer sources than most people expect. Never take large doses of over-the-counter potassium on your own; the dose and follow-up testing should come from your clinician.
Do I need to go to the ER for a potassium of your value?
Not for this value by itself. It belongs in a conversation with your clinician — same-day for clearly abnormal results, routine for borderline ones — but if you ever develop palpitations, chest pain, marked muscle weakness, or trouble breathing alongside an abnormal potassium, seek emergency care regardless of the number.
Which medicines change potassium levels?
Plenty of common ones do. Medicines that raise potassium include ACE inhibitors (lisinopril), ARBs (losartan), spironolactone and other potassium-sparing diuretics, NSAIDs (ibuprofen, naproxen), and the antibiotic trimethoprim. Medicines that lower it include thiazide and loop diuretics (hydrochlorothiazide, furosemide), insulin, albuterol inhalers, corticosteroids, and overused laxatives. Potassium-based salt substitutes and supplements push levels up, sometimes sharply in people with kidney disease. Never stop or change a prescription because of one lab value — bring the list to your clinician and adjust together.
Browse Potassium values
Sources
- Potassium Blood Test — MedlinePlus (National Library of Medicine)
- Hypokalemia (StatPearls) — NCBI Bookshelf / StatPearls
- Hyperkalemia (StatPearls) — NCBI Bookshelf / StatPearls
- Low potassium (hypokalemia) — Mayo Clinic
- High potassium (hyperkalemia) — Mayo Clinic
- Potassium and Your Diet — National Kidney Foundation
Educational information, not medical advice. Full disclaimer