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Magnesium: Are We Consuming Enough?
Nutrients. 2018 Dec; 10(12): 1863.

Magnesium is essential for maintaining normal cellular and organ function. In-adequate magnesium balance is associated with various disorders, such as skeletal deformities, cardiovascular diseases, and metabolic syndrome. Unfortunately, routinely measured serum magnesium levels do not always reflect total body magnesium status. Thus, normal blood magnesium levels eclipse the wide-spread magnesium deficiency. Other magnesium measuring methods, including the magnesium loading test, may provide more accurate reflections of total body magnesium status and thus improve identification of magnesium-deficient individuals, and prevent magnesium deficiency related complications.

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From the above review paper.

“In humans, red blood cell (RBC) magnesium levels often provide a better reflection of body magnesium status than blood magnesium levels. When the magnesium concentration in the blood is low, magnesium is pulled out from the cells to maintain blood magnesium levels within normal range. Therefore, in case of magnesium deficiency, a blood test of magnesium might show normal levels, while an RBC magnesium test would provide a more accurate reflection of magnesium status of the body. For exact estimation of RBC magnesium level, individuals are advised not to consume vitamins, or mineral supplements for at least one week before collection of RBC samples. A normal RBC magnesium level ranges between 4.2 and 6.8 mg/dL. However, some experts recommend aiming for a minimum level of 6.0 mg/dL on the RBC test.”