Electrolytes blood test: potassium, sodium, magnesium, calcium ranges

An electrolyte blood test measures the charged minerals that keep fluid, nerve and muscle function working — most commonly sodium, potassium, chloride and bicarbonate (CO2), reported together as a basic panel. Calcium and magnesium are related electrolytes many labs report separately: calcium is often part of a broader metabolic panel, and magnesium is frequently ordered on its own, such as when potassium is unexpectedly low.

This article covers what a routine electrolyte panel includes, the reference range for each analyte, the common causes behind a high or low result, and the red flags that need prompt attention. It does not repeat the cardiovascular risk picture already covered in our heart health blood tests guide, the kidney-specific detail in our kidney function tests guide, or the calcium/PTH deep dive in our calcium and PTH bone health guide.

Electrolyte reference ranges

*Calcium range from Testing.com, ages 20–49, by sex; limits shift slightly at other ages and by lab (mg/dL ÷ 4.008 for the mmol/L conversion). †Magnesium threshold from MSD Manual (Hypomagnesemia), mg/dL and mmol/L as that source publishes them. Every lab prints its own reference range on the report, and that is the one that applies to your result — treat the numbers above as a general guide, not a substitute for it.

Per Testing.com, a standalone electrolyte panel typically does not require fasting, since these values are not significantly affected by a recent meal the way glucose is; if ordered alongside a fasting test such as a lipid panel, follow the fasting instructions for that other test.

Potassium: what pushes it up or down

Potassium sits mostly inside cells, so even a small shift in the blood level can matter for the heart. The MSD Manual defines hyperkalemia as a potassium level above 5.5 mmol/L, which is usually asymptomatic until cardiac toxicity develops and, if severe, can progress to ventricular fibrillation or asystole. Recognised causes include chronic or acute kidney disease; ACE inhibitors, angiotensin receptor blockers (ARBs) and potassium-sparing diuretics such as spironolactone, which the manual groups together as medications that reduce potassium excretion; and potassium supplementation. Testing.com adds that ACE inhibitor use and kidney disease are among the everyday reasons a result comes back high.

Hypokalemia — potassium below 3.5 mmol/L, per the MSD Manual — has a different set of drivers. Diuretics, especially loop and thiazide types, are the most common medication cause. Testing.com notes low potassium is also common with vomiting or diarrhoea, since gastrointestinal losses push potassium down. Low magnesium is a further contributor: the MSD Manual states “hypomagnesemia is a common correlate of hypokalemia,” and correcting the magnesium is often necessary before potassium stabilises. Symptoms include muscle weakness, cramping and palpitations, and — like hyperkalemia — significant hypokalemia can trigger dangerous heart rhythm disturbances.

One more thing worth real attention: how the sample was drawn can distort the result before it ever reaches you. Pseudohyperkalemia — a falsely high reading that does not reflect your actual blood level — most often comes from hemolysis, meaning red blood cells break open during or after the draw and release their potassium into the sample. The MSD Manual also names prolonged tourniquet use and excessive fist clenching during venipuncture as causes. Testing.com makes the same point in plain terms: a “hemolyzed (broken-down) blood sample” can produce a high reading, which is why a very high result is often repeated before anyone acts on it. If your potassium comes back unexpectedly high and you feel well, ask whether the sample was hemolyzed and a repeat draw is worth doing first.

Sodium: dehydration, diuretics, heart failure, and a term to recognise but not self-diagnose

Sodium is reported as low (hyponatremia) when it falls below roughly 135–136 mmol/L, values close together across Testing.com’s typical range and the MSD Manual’s clinical threshold. Testing.com’s electrolyte panel guide notes diuretics — thiazide types in particular — lower both sodium and potassium, one reason people on these medications get periodic electrolyte panels.

The MSD Manual describes several distinct routes to low sodium: dehydration and gastrointestinal fluid losses replaced with plain water; heart failure and liver disease, which reduce effective circulating blood volume and trigger hormone changes that retain water out of proportion to sodium; and diuretic use, especially in older adults. A separate cause the manual names is SIADH — the syndrome of inappropriate antidiuretic hormone secretion, where the body releases too much of the hormone that controls water retention even though blood volume is normal. SIADH is not something to work out from a single sodium number; it is diagnosed and managed by a clinician, not inferred from a home reading of the lab report.

High sodium (hypernatremia) usually points the other way: not enough fluid intake relative to losses — a hydration question rather than a diet one.

Magnesium: often overlooked, and linked to potassium

Magnesium is frequently ordered separately from the basic electrolyte panel, often because potassium is unexpectedly low or will not correct. Per the MSD Manual, hypomagnesemia is a serum magnesium level below roughly 1.8 mg/dL (0.70 mmol/L). Common causes include diuretics — loop and thiazide types both increase urinary magnesium losses — long-term proton pump inhibitor (PPI) use, which impairs absorption; chronic alcohol use, through reduced intake and increased kidney losses; and diabetes, where diabetic ketoacidosis can trigger magnesium depletion.

The connection to potassium runs in both directions: low magnesium makes it harder for the kidneys to hold on to potassium, so hypokalemia caused by low magnesium often will not fully correct with potassium replacement alone until the magnesium deficit is addressed. This is why a clinician investigating unexplained or treatment-resistant low potassium will often check magnesium at the same time.

Calcium: one piece of a larger picture

Total calcium in the blood runs 8.6–10.3 mg/dL (2.15–2.57 mmol/L) in men and 8.6–10.2 mg/dL (2.15–2.54 mmol/L) in women aged 20–49, per Testing.com, with slightly different limits at other ages — the range on your own lab report is the one to use. One detail worth knowing: total calcium is affected by your albumin level, and Testing.com notes “low albumin can make total calcium look lower than it actually is,” which is why a provider may calculate a corrected calcium value when albumin is abnormal. Calcium is also interpreted together with parathyroid hormone (PTH) rather than alone — for that fuller picture, see our calcium and PTH bone health guide; this article does not repeat that interpretation.

Who typically gets an electrolyte panel

Testing.com lists the everyday reasons this panel gets ordered: an existing diagnosis of kidney disease, heart failure or high blood pressure; regular use of diuretics, ACE inhibitors, corticosteroids or laxatives; and symptoms such as unusual weakness, cramping or dizziness. People with an arrhythmia or a recent episode of prolonged vomiting or diarrhoea are also commonly tested. The broader hypertension work-up is a separate topic, not covered here.

Preparation and factors that distort the result

Fasting is generally not required for a standalone electrolyte panel — Testing.com notes these levels “don’t change meaningfully after eating” the way glucose does — but if it is drawn alongside a fasting test, follow the fasting instructions for that other test. Tell whoever draws your blood about all medications and supplements you take: Testing.com notes diuretics lower potassium and sodium, ACE inhibitors and ARBs raise potassium, and corticosteroids raise sodium while lowering potassium, so a result is read alongside your medication list, not as a number on its own.

Beyond medications, sample handling matters. As covered above for potassium, hemolysis during the draw, a prolonged tourniquet, or repeated fist clenching can push a reading artificially high without reflecting your actual blood chemistry. If a result looks out of step with how you feel, asking whether the sample quality was normal is reasonable before assuming the number is accurate.

Red flags: when not to wait

If your result is far outside the reference range, the lab or the clinician who ordered the test will usually contact you directly, and you do not need to wait for a scheduled follow-up. Testing.com advises seeking immediate medical attention for trouble breathing, seizures, or similar acute symptoms alongside an abnormal result. More generally, palpitations, marked new muscle weakness, or confusion together with a known or suspected abnormal potassium, sodium, magnesium or calcium result should prompt urgent care rather than waiting — these are the patterns the sources above link to dangerous heart rhythm or neurological effects of electrolyte imbalance.

Tracking electrolytes with HealthLab

Electrolyte results are easiest to interpret next to each other over time, particularly for someone on a diuretic whose potassium and kidney function are checked together at intervals. HealthLab can import a lab PDF or photo using AI recognition, and results can also be entered manually with an editable unit and your own lab’s reference range — the range that matters, as noted above. Manual entry is free and unlimited; one import is free, unlimited import is Pro. Each analyte gets its own trend chart, so a potassium trend can sit next to a creatinine trend for someone monitored on a diuretic, and results export to PDF ahead of an appointment. Additional profiles for family members come with Pro.

For the full list of what HealthLab tracks, see the lab results page. HealthLab organises results and helps you prepare for the conversation with a doctor; it does not diagnose a condition or decide whether a result needs treatment.

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