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Magnesium, RBC

Magnesium measured inside your red blood cells. Some clinicians use it to look at magnesium stores, but how well it reflects your whole-body magnesium is debated.

AI-drafted. Awaiting verificationAlso called RBC Mg, Mg RBCLOINC 26746-8

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What it is

Magnesium helps your muscles, nerves and heart work, and most of it sits inside cells and bone rather than in the fluid part of your blood. The usual blood test measures magnesium in serum. This test measures it inside red blood cells instead. Red cells hold more magnesium than serum does, so some clinicians look at this level as a window on magnesium inside cells. It is not on routine panels, it is not standardized between labs, and labs print different ranges.

Why it matters

Low magnesium can cause muscle cramps, tremor, weakness or an uneven heartbeat, and it often goes along with low potassium or calcium. Serum magnesium reflects only a small part of the body's supply, so some clinicians add an RBC magnesium test when symptoms or risk factors suggest low stores. The evidence for this test is limited. Reviews note that few well-designed studies show RBC magnesium reliably reflects total body magnesium, and the National Institutes of Health says no single method is considered satisfactory. A result is best read together with serum magnesium, your symptoms, diet, medicines and kidney function.

If it’s low

A result below your lab's range may mean magnesium inside your cells is lower than usual. On its own it does not show a deficiency, because how well this test reflects whole-body magnesium is debated. The usual causes of low magnesium are low intake, long-term diarrhea or poor absorption, heavy alcohol use, poorly controlled diabetes and some medicines. Your clinician can put your result in context, usually alongside serum magnesium, potassium, calcium and kidney tests.

Common causes

  • Low dietary intake of magnesium
  • Long-term diarrhea, or bowel conditions or surgery that reduce absorption
  • Water pills (loop and thiazide diuretics)
  • Long-term use (over a year) of acid-reducing medicines called proton pump inhibitors
  • Heavy alcohol use
  • Poorly controlled diabetes
  • Some other medicines, such as certain antibiotics (amphotericin B) and cisplatin

What can help

  • Ask your clinician whether serum magnesium, potassium, calcium and kidney function should be checked too
  • Ask whether a medicine, diarrhea, diet or alcohol use could explain it
  • Eat magnesium-rich foods such as dark leafy greens, nuts, seeds, beans and whole grains
  • Talk with your clinician before taking magnesium supplements, especially if you have kidney disease

If it’s high

A result above your lab's range is uncommon, and there is no single accepted explanation for it. The usual reasons for high magnesium in serum are regular use of magnesium-containing antacids, laxatives or supplements and reduced kidney function, which lowers how well the body clears magnesium. Red-cell magnesium has been studied far less, so it is not clear that the same reasons apply. Your clinician can put your result in context, usually alongside serum magnesium and kidney tests.

Common causes

  • Regular use of magnesium supplements, antacids or laxatives
  • Reduced kidney function, which lowers how well the body clears magnesium
  • Differences between labs, since the test is not standardized

What can help

  • Tell your clinician about every supplement, antacid and laxative you use
  • Ask whether serum magnesium and kidney function should be checked

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 3.7–7 mg/dL. Source: Labcorp, NIH Office of Dietary Supplements, Nutrients, Merck Manual Professional Version.

What can affect your result

  • Sample type: only red cells are tested, and serum or plasma samples are rejected (Labcorp)
  • Red cells turn over slowly, so the result reflects weeks to months rather than days
  • Diet, supplements and medicines that change magnesium balance
  • Kidney function, which controls how much magnesium the body keeps
  • Lab and method: the test is not standardized, so ranges differ between labs
  • This is a different test from serum magnesium, with a different range (for example 1.7–2.3 mg/dL for adults at Mayo Clinic Laboratories)
  • Only red blood cells are tested. Labcorp asks that the plasma be removed and discarded, and it rejects serum or plasma samples sent in place of red cells.
  • Labcorp accepts a green-top (heparin), lavender-top (EDTA), royal blue-top (EDTA) or tan-top lead-free tube, spun down promptly, with the red cells sent in the original tube.
  • Magnesium supplements, magnesium-containing antacids or laxatives and some medicines can change how much magnesium your body holds, so tell your clinician about everything you take.

Slow. Red blood cells live for months, and studies that linked RBC magnesium to intake used diets lasting about 3 months.

When to talk to a clinician

Contact the clinician who ordered the test if your result is outside your lab's range, especially with muscle cramps, tremor, weakness, palpitations or unusual drowsiness. No published critical value was found for this test. Call 911 or your local emergency number for chest pain, a seizure, fainting, or very slow or shallow breathing.

Questions you could ask

  • Does this test add anything beyond a serum magnesium level for me?
  • Should it be repeated, and when?
  • Could a supplement, antacid or laxative I use explain a higher result?
  • Should we check my kidney function and serum magnesium?
  • Could my diet, digestion, alcohol use or a medicine explain a lower result?
  • Should we also check serum magnesium, potassium and calcium?

Questions people ask

Is RBC magnesium better than a regular magnesium test?

Not clearly. Red cells hold more magnesium, so some clinicians use this test to look inside cells. But reviews say there is too little good evidence to call it more reliable, and the National Institutes of Health says no single test is considered satisfactory. Serum magnesium is still the usual first test.

Why does my lab's range differ from ranges I see online?

RBC magnesium is not standardized between labs, and each lab sets its own range. Labcorp, for example, prints 3.7–7.0 mg/dL. Other labs and websites give different numbers. Use the range on your own report.

How do I convert RBC magnesium between mg/dL and mmol/L?

Multiply mg/dL by 0.411 to get mmol/L, so 5.0 mg/dL is about 2.06 mmol/L. Multiply mmol/L by 2.43 to go the other way.

Show the science and sources

Units

  • 1 mg/dL = 1 mg/dL
  • 1 mg/dL = 0.4114 mmol/L

Open the RBC Magnesium unit converter

Is a change real?

RBC Magnesium naturally varies about 5.6% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 17% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Desirable Biological Variation Database specifications, 2014 update (Minchinela J, Ricos C, et al.; the 'Ricos database', a compilation of median published CVI/CVG).

For clinicians and researchers

Body magnesium is about 0.8% blood (0.3% serum and 0.5% erythrocytes), with the rest in soft tissue, muscle and bone (Workinger et al. 2018). Red cells hold more magnesium than serum, so RBC magnesium is often cited as preferable to serum, and some studies show correlation with magnesium status after long-term (about 3-month) replete or deplete diets. However, most RBC studies do not meet that design, there are few randomized trials, and most do not check the result against other compartments such as urine or muscle, so the claim that RBC magnesium represents the large magnesium pool is not well supported (Workinger 2018). The NIH Office of Dietary Supplements notes that serum magnesium is the most commonly used measure even though it correlates poorly with total body or tissue levels, that erythrocyte, saliva, urine and ionized magnesium measurements and the magnesium-loading test are alternatives, and that no single method is considered satisfactory. Labcorp offers RBC magnesium (test 080283) as a laboratory-developed test measured by atomic absorption spectrometry or ICP/MS on red cells separated from plasma, with a range of 3.7–7.0 mg/dL (about 1.52–2.88 mmol/L); because the test is not standardized, intervals from other labs differ. Serum magnesium (separate marker) has limits of its own and is the usual first test.

LOINC 26746-8 (primary), 2597-3. Specimen: red-blood-cells.

Sources

  1. Magnesium, RBC (test 080283), Labcorp
  2. Magnesium, RBC (test 080283), sample patient report, Labcorp (2024)
  3. Magnesium: Fact Sheet for Health Professionals, NIH Office of Dietary Supplements
  4. Challenges in the Diagnosis of Magnesium Status, Nutrients (2018)
  5. Hypomagnesemia, Merck Manual Professional Version
  6. Hypermagnesemia, Merck Manual Professional Version
  7. Magnesium (PubChem compound CID 5462224), National Library of Medicine, PubChem

AI-drafted. Awaiting verification. Last reviewed 2026-10-03. How we build and check these pages.

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