Type the potassium result from your blood test. You see where it sits against the range the NIH calls normal and the range MedlinePlus prints, what the two sources say moves the number, and when to call. Your lab’s own range, and your doctor, have the last word.
4.2 mmol/L is
Inside ODS’s normal range 4.2 mmol/L = 4.2 mEq/L
4.2 mmol/L: inside ODS’s normal range (3.6–5.0 mmol/L), and inside MedlinePlus’s 3.7–5.2 mEq/L.
That is inside the range NIH ODS calls normal. The range printed on your own lab report is the one your doctor will read it against.
What moves the number, in the sources’ words
- Otherwise, in healthy individuals with normal kidney function, abnormally low or high blood levels of potassium are rare.
| Line | mmol/L |
|---|---|
| NIH ODS: moderate to severe hypokalemia | under about 2.5 |
| NIH ODS: hypokalemia | below 3.6 |
| NIH ODS: normal | about 3.6 to 5.0 |
| NIH ODS: hyperkalemia | above 5.0 |
| MedlinePlus potassium test: normal | 3.7 to 5.2 |
| MedlinePlus low potassium page: normal | 3.5 to 5.1 |
What a blood result can and cannot tell you. NIH ODS: “Although blood potassium levels can provide some indication of potassium status, they often correlate poorly with tissue potassium stores.” MedlinePlus: “Normal value ranges may vary slightly among different laboratories.”
This is not medical advice and it does not diagnose anything. It shows where a number sits against published ranges; what your result means for you depends on your lab, your medicines, your kidneys, your history and your doctor.
NIH’s Office of Dietary Supplements: “Normal serum concentrations of potassium range from about 3.6 to 5.0 mmol/L” — hypokalemia is below 3.6, hyperkalemia above 5.0. MedlinePlus prints a lab’s range of 3.7 to 5.2 mEq/L. For potassium, mmol/L and mEq/L are the same number, so there is nothing to convert.
How to use it
Find “Potassium” (or “K”) on your blood test — it is usually in the basic or comprehensive metabolic panel — and type the number. You see the band it falls in against NIH ODS’s lines, whether it is inside MedlinePlus’s range, what the two sources say moves the number, and when MedlinePlus says to call. Where the two ranges disagree, the page says so instead of choosing. It does not tell you what to take: that is the conversation with your doctor.
The lines, and why there is no conversion
NIH ODS (Potassium fact sheet, updated June 2, 2022) gives the normal range and both lines: “hypokalemia (serum levels below 3.6 mmol/L) or hyperkalemia (serum levels above 5.0 mmol/L)”. It adds a lower line: “Moderate to severe hypokalemia (serum potassium level less than about 2.5 mmol/L)”. Neither ODS nor MedlinePlus prints a number for a severe high result, so this page does not invent one. MedlinePlus’s potassium test page (review date 5/19/2025): “The normal range is 3.7 to 5.2 milliequivalents per liter (mEq/L)”, and “Normal value ranges may vary slightly among different laboratories.” Its own low-potassium page prints a third range — “The normal range is 3.5 to 5.1 mEq/L (3.5 to 5.1 mmol/L).” — which is the clearest proof there is that the range on your report is the one that counts. The units are simple: potassium carries one charge, so ODS’s “1 mmol = 1 milliequivalent [mEq] or 39.1 mg potassium” makes 4.2 mmol/L and 4.2 mEq/L the same result.
| Result | NIH ODS | MedlinePlus test page, 3.7–5.2 | MedlinePlus low-potassium page, 3.5–5.1 |
|---|---|---|---|
| 2.4 | Under its moderate-to-severe line | Under | Under |
| 3.0 | Under its normal range | Under | Under |
| 3.5 | Under its normal range | Under | Inside |
| 3.6 | Inside its normal range | Under | Inside |
| 4.2 | Inside its normal range | Inside | Inside |
| 5.0 | Inside its normal range | Inside | Inside |
| 5.1 | Above its normal range | Inside | Inside |
| 5.3 | Above its normal range | Over | Over |
| 6.0 | Above its normal range | Over | Over |
What moves the number
NIH ODS: “Diarrhea, vomiting, kidney disease, use of certain medications, and other conditions that alter potassium excretion or cause transcellular potassium shifts can cause hypokalemia (serum levels below 3.6 mmol/L) or hyperkalemia (serum levels above 5.0 mmol/L).” And: “Otherwise, in healthy individuals with normal kidney function, abnormally low or high blood levels of potassium are rare.” On the low side, “Hypokalemia is rarely caused by low dietary potassium intake alone, but it can result from diarrhea due to potassium losses in the stool.” On the high side, ODS names “chronic kidney disease or the use of certain medications, such as angiotensin converting enzyme (ACE) inhibitors or potassium-sparing diuretics”.
MedlinePlus lists what a high result may be due to: Addison disease (rare); Blood transfusion; Certain medicines, including angiotensin converting enzyme (ACE) inhibitors, angiotensin receptor blockers (ARBs), the potassium-sparing diuretics spironolactone, amiloride and triamterene, and beta adrenergic blockers; Crushed tissue injury; Hyperkalemic periodic paralysis; Hypoaldosteronism (very rare); Kidney insufficiency or failure; Metabolic or respiratory acidosis; Red blood cell destruction; Too much potassium in your diet. And a low result: Acute or chronic diarrhea; Cushing syndrome (rare); Diuretics such as hydrochlorothiazide, furosemide, torsemide, and indapamide; Hyperaldosteronism; Hypokalemic periodic paralysis; Not enough potassium in the diet; Renal artery stenosis; Renal tubular acidosis (rare); Vomiting. Which of these, if any, applies to you is your doctor’s question, not a list’s.
One reason has nothing to do with your body. MedlinePlus: “If it is hard to get the needle into the vein to take the blood sample, injury to the red blood cells may cause potassium to be released. This may cause a falsely high result.”
Worked examples
- 4.2 mmol/L (the page’s example) is inside ODS’s range and both of MedlinePlus’s.
- 3.65 mmol/L is inside ODS’s 3.6 to 5.0 but under MedlinePlus’s 3.7. That is where the ranges disagree, and the page says so rather than choosing.
- 5.1 mEq/L is above ODS’s 5.0 line and inside MedlinePlus’s 3.7 to 5.2 — and the page adds MedlinePlus’s note that a hard blood draw can read falsely high.
- 2.4 mmol/L is under ODS’s moderate-to-severe line, and the page quotes ODS — “Severe hypokalemia can be life threatening” — and says to call your doctor promptly.
Common mistakes
- Reading the number without your lab’s range. Even MedlinePlus prints two different ranges; your report prints its own.
- Forgetting how the blood was drawn. A difficult draw can release potassium from red cells and read falsely high.
- Treating a low result with a supplement. ODS: hypokalemia is “rarely caused by low dietary potassium intake alone”. Over-the-counter pills hold up to 99 mg (see why), and whether you need more is your doctor’s call.
- Adding potassium on kidney disease or certain blood-pressure medicines. ODS: with chronic kidney disease, ACE inhibitors or potassium-sparing diuretics, “even dietary potassium intakes below the AI can cause hyperkalemia”.
Frequently asked questions
What are normal potassium levels?
NIH ODS: “Normal serum concentrations of potassium range from about 3.6 to 5.0 mmol/L.” MedlinePlus’s potassium test page prints 3.7 to 5.2 mEq/L, and its low-potassium page 3.5 to 5.1. Your lab’s own range is the one your doctor uses.
What is the potassium normal range in mEq/L?
The same numbers as in mmol/L: potassium carries one charge, so ODS’s “1 mmol = 1 milliequivalent [mEq]” holds. ODS’s 3.6 to 5.0 mmol/L is 3.6 to 5.0 mEq/L.
What potassium level is too low?
ODS calls a serum level below 3.6 mmol/L hypokalemia, and less than about 2.5 mmol/L moderate to severe. A low result is a conversation with your doctor; under 2.5, a prompt one.
What potassium level is too high?
ODS calls a serum level above 5.0 mmol/L hyperkalemia. Neither ODS nor MedlinePlus prints a number for a severe high result. MedlinePlus: “Contact your provider right away if you have vomiting, palpitations, weakness, or difficulty breathing, or if you’re taking a potassium supplement and have symptoms of high potassium.”
Can a potassium blood test be wrong?
It can read high for a reason outside your body. MedlinePlus: “If it is hard to get the needle into the vein to take the blood sample, injury to the red blood cells may cause potassium to be released. This may cause a falsely high result.” Your doctor may repeat the test.
What causes low potassium levels?
ODS names diarrhea, vomiting, kidney disease and certain medications, and adds that hypokalemia “is rarely caused by low dietary potassium intake alone”. MedlinePlus’s list includes diuretics such as hydrochlorothiazide and furosemide. Which applies to you is your doctor’s question.
Does a normal result mean I get enough potassium?
Not necessarily. NIH ODS: “Although blood potassium levels can provide some indication of potassium status, they often correlate poorly with tissue potassium stores.” Intake is a separate question — the Potassium Supplement Calculator places a day’s supplement potassium against the adequate intake.
Read next
- The Potassium Supplement Calculator — the intake side of the same mineral — and the Magnesium Level Checker — magnesium is one of the tests MedlinePlus says may be ordered beside a low potassium.
- Multivitamins for adults: how to choose the right complex
- Do nootropics raise blood pressure?
- Colon Broom vs Zupoo: why are they popular?
- How to get rid of anemia? The best 4 products
This is not medical advice, and nothing here diagnoses or treats anything. It shows where a number sits against published ranges; what it means for you is a conversation with your doctor, who knows your lab, your medicines and your history. Some links on this site are affiliate links: I buy what I review at full price, and a commission never changes a verdict.