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Serum magnesium may be more common, but is red blood cell magnesium a better test?

Updated: Jul 24, 2023

Board with Mg written surrounded by food.
Dietary sources of magnesium

Magnesium Functions

We know that magnesium is needed by every organ of the body and also affects hundreds of metabolic functions within the body. In addition, magnesium has very important antioxidant and anti-inflammatory functions. But is it at sufficient levels to get to where it needs to be? Measuring magnesium in the blood (serum magnesium) in general can provide an idea of what is available. However, measuring magnesium within the cell, the red blood cell (RBC) magnesium level, gives us an idea of what is actually ready to be used.

Both values are useful for assessing magnesium status, especially since a normal serum magnesium does not necessarily rule out insufficiency.

Serum levels may be maintained within normal range at the expense of releasing magnesium from the bone which can then deplete skeletal magnesium. On the other hand, serum levels can increase during excess breakdown of red blood cells where magnesium concentration is usually three times that of serum.

Magnesium Levels

Magnesium insufficiency can lead to high blood pressure, cardiac arrhythmias, cardiovascular disease, osteoporosis, headaches, depression, sleep disturbances, other neurological functions, muscle cramps, impaired glucose metabolism and regulation, kidney stones, and more. Medications that can decrease levels include diuretics, insulin, and some antibiotics (Pagana 2021).

Elevated levels of magnesium are uncommon but can occur with excess intake (particularly now when many people take magnesium supplements without even knowing their level or risk for deficiency). Increased levels can also be caused by insufficient excretion by the kidneys or medications such as magnesium-containing antacids, laxatives, lithium, loop diuretics, thyroid medication, and aminoglycoside antibiotics (Pagana 2021). Disease states that can increase magnesium levels include: uncontrolled diabetes, hypothyroidism, renal insufficiency/decreased excretion, slower cardiac conduction, and Addison disease.

High magnesium levels can be associated with hypotension, tachycardia, muscle weakness, and even cardiac arrest.

Lab Range

Measuring RBC magnesium levels, as opposed to serum, gives us a better idea of what is available intracellularly and ready to be used (Razzaque 2018).

Conventional Lab Range: 4.00 - 6.80 mg/dL (1.66 - 2.82 mmol/L)

A better Functional Lab Range: 5.7-6.2 mg/dL

Concerns with Low Magnesium

Magnesium insufficiency increases the risk of metabolic disorders, including CVD, hypertension, type 2 diabetes, osteoporosis, musculoskeletal, and neurological disorders (Costello 2016), including anxiety and depression.

Although evaluation of serum magnesium assists in identifying insufficiency, measurement of red blood cell magnesium may provide additional information about what is truly available and may better reflect body stores.

As previously mentioned, the magnesium concentration in red blood cells is three times that of serum. Therefore, in severe hemolysis, a significant amount of magnesium can be released into the blood, (falsely) increasing serum levels (Cascella 2022).

To conclude, RBC magnesium is more indicative of true magnesium status than serum magnesium levels and should be the preferred test ran when given the option between the two tests.

Practitioners who would like to learn more about magnesium, both RBC and serum, and over 60 other blood biomarkers, enroll in the lab course Interpret Normal Labs for Optimal Health.


Cascella, Marco. and Sarosh Vaqar. “Hypermagnesemia.” StatPearls, StatPearls Publishing, 30 May 2022.

Pagana, Kathleen Deska, et al. Mosby's Diagnostic and Laboratory Test Reference. 15th ed., Mosby, 2021.

Razzaque, Mohammed S. “Magnesium: Are We Consuming Enough?.” Nutrients vol. 10,12 1863. 2 Dec. 2018, doi:10.3390/nu10121863


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