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Magnesium in Urine

While more than 60% of the body’s magnesium stores reside in bone, nearly 70% of the serum magnesium circulates freely, not bound to plasma proteins. This unbound magnesium is freely filtered at the glomerulus and enters the tubules where more than 95% of filtered magnesium is reabsorbed in the loop of Henle in the kidney. 

Measurement of urine magnesium is helpful in the evaluation of hypomagnesemia in order to differentiate renal vs nonrenal losses. A 24-hour urine collection is preferred over a spot sample because magnesium excretion varies with dietary intake and circadian rhythm. A 24-hour collection also is useful in detecting polyuria which can reduce tubular absorption of magnesium. Magnesium supplements and diuretics should be discontinued prior to urine collection.

If a 24-hour urine cannot be collected, then fractional excretion of magnesium (FEMg) must be calculated to adjust urinary magnesium for urinary creatinine.

FEMg = Serum Creatinine × Urine Magnesium/0.7 × Serum Magnesium × Urine Creatinine

In the formula, serum magnesium is multiplied by 0.7 in the denominator to account for the fraction not bound to albumin.

Renal magnesium wasting is defined as a daily urinary magnesium excretion of more than 1 mmol per day (24.31 mg/day) or a fractional excretion of magnesium greater than 4%.

Common causes of renal wasting of magnesium include:

  • Diuretics
  • Antimicrobials (amphotericin B, aminoglycosides) Calcineurin inhibitors (cyclosporine, tacrolimus) 
  • Epidermal growth factor receptor inhibitors (cetuximab) 
  • Cytotoxic drugs (cisplatin, carboplatin)
  • Hypercalcemia
  • Hyperthyroidism, hyperparathyroidism
  • Alcohol use
  • Diabetes 

Reference

Tucker BM, et al. Urinary Magnesium in the Evaluation of Hypomagnesemia, JAMA Published online October 30, 2020.

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