Normally the body excretes any excess magnesium into the urine. If the patient is deficient, then more is retained to replaced depleted stores.
Patient selection: adequate renal function
Infusion 0.2 mmol magnesium sulfate per kg body weight, infused over a 4-hour period
Molecular weight of magnesium sulfate anhydrous: 120.37 g
Protocol for magnesium loading test:
(1) magnesium concentration in a 24-hour urine completed prior to the infusion
(2) magnesium concentration in a 24-hour urine following the infusion
magnesium retention =
= 1 - (((postinfusion magnesium excretion) - (preinfusion magnesium excretion)) / (total magnesium infused)) * 100
Interpretation:
• A retention >= 50% indicates magnesium deficiency.
