Serum magnesium levels may not reflect total body magnesium stores. Urinary excretion of magnesium can help identify a patient who may benefit from closer monitoring and replacement therapy.


molecular weight of magnesium: 24.305

valence of magnesium ion: 2 (1 mmol = 2 mEq)


Patient selection: one or more risk factors for poor intake, poor absorption or increased loss


24 hour urine output for magnesium in mg =

= (magnesium concentration in mg/dL) * (24 hour urine output in mL) / 100


24 hour urine output for magnesium in mmol =

= (magnesium concentration in mmol/L) * (24 hour urine output in mL) / 1000



• The lower limit of the normal reference range for magnesium excretion is 3 mmol/day (approximately 75 mg/day).

• A decreased urine output but normal or low normal magnesium reflects decreased total body stores.

• Low serum magnesium levels usually are associated with a urine excretion below 20-25 mg/day (0.82 - 1.03 mmol/day; see Figure 2, page 23)


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