Serum magnesium, like serum potassium, is often normal despite depletion of total body magnesium Intracellular levels of magnesium are more accurate measures; however, there is often poor correlation of intracellular red blood cell and intracellular mononuclear cell magnesiumand these may correlate poorly with the magnesium content of muscle in different disease states Of the two, intracellular mononuclear cell magnesium is a better indicator of the magnesium status of the heart For example, in a recent study of patients in the coronary care unit, only 7.7% had low serum magnesium levels, but 53% showed low levels of mononuclear cell magnesium.

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