How are magnesium levels measured?

Updated: Oct 30, 2020
  • Author: Tibor Fulop, MD, PhD, FACP, FASN; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

Patients whose clinical condition suggests magnesium depletion should have their serum magnesium level measured. Most laboratories use a reference range of 1.5 to 2.5 mg/dL for serum magnesium. Hypomagnesemia reliably indicates reduced total body stores of magnesium.

However, while measurement of serum magnesium is relatively easy, and it has become the method of choice to estimate magnesium content, its ability to evaluate total body stores of magnesium is limited. A person may be intracellularly magnesium depleted and exhibit signs of magnesium deficiency, yet have normal serum levels of magnesium. In this setting, serum magnesium tends to remain within the normal range due to recruitment of intracellular stores, until the point where the intracellular stores cannot keep up.

Unfortunately, no quick, simple, and accurate test is available to measure magnesium at the intracellular level, which is where the major physiologic role of magnesium occurs. Excluding magnesium deposited in bone, which is poorly mobilized, the extracellular fluid space contains only 2% of total body magnesium, and levels there may not always accurately reflect the intracellular magnesium status.


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