When is measurement of magnesium retention used in the diagnosis of hypomagnesemia?

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

A surrogate for direct intracellular magnesium is the measurement of magnesium retention after acute magnesium loading. A magnesium deficiency is indicated if a patient has reduced excretion (< 80% over 24 h) of an infused magnesium load (2.4 mg/kg of lean body weight given over the initial 4 h). 

This method is useful only when the clinical suggestion of magnesium deficiency is strong (eg, the patient has unexplained cardiovascular or neuromuscular abnormalities) but serum magnesium levels are normal. It is not helpful in the setting of renal magnesium wasting (as seen with diuretics) or in the presence of renal dysfunction.

However, the utility of this test is uncertain. Patients with malnutrition, cirrhosis, diarrhea, or long-term diuretic use typically have a positive result, whether or not they have signs or symptoms referable to magnesium depletion. It seems prudent, therefore, to simply administer magnesium to these patients if they have unexplained hypocalcemia and/or hypokalemia.

Another caveat to estimating serum magnesium levels is that although only free magnesium is biologically active, most methods of assessing the serum content measure total magnesium concentration. Because 30% of magnesium is bound to albumin and is therefore inactive, hypoalbuminemic states may lead to spuriously low magnesium values.


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