Diagnosing magnesium deficiency can be difficult. The amount of a loading dose that is excreted in the urine reflects the magnesium body stores. A person with normal stores will excrete almost all of a loading dose, while a deficient person will retain some, resulting in decreased excretion.



(1) The urinary bladder is emptied in the morning.

(2) A loading dose of 30 mmol magnesium sulfate in 1,000 mL isotonic saline is administered IV as a continuous infusion over 8 hours.

(3) A 24 hour urine is collected with 15 mL of 10% hydrochloric acid as additive.

(4) The amount of magnesium in the urine is calculated.

(5) To simplify calculations, dietary intake, basal urinary and basal fecal excretion of magnesium are ignored.



• The molecular weight for magnesium sulfate (MgSO4) is 120.4 if anhydrous.

• Since magnesium is bivalent, 1 mol = 2 equivalents, so 1 mmol = 2 mEq


magnesium excreted in 24 hour urine =

= (mmol/L of magnesium in urine) * (24 hour urine volume in L)


percent of magnesium excreted =

= 100% * ((mmol of magnesium excreted in 24 hour urine) / (mmol of magnesium administered))


magnesium retention =

= 100% - (percent of magnesium excreted)



• A normal person will excrete most of the magnesium load during the testing period.

• Based on control populations, the reference range for retention is –23% to + 29%, which is the mean +/- 2 standard deviations.

• Magnesium deficiency was defined as a magnesium retention > 29%.

• Patients with a high frequency of magnesium deficiency include chronic alcoholics, diabetics and patients with cardiovascular disorders.


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