Protocol:
(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.
where:
• 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)
Interpretation:
• 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.
