The Fractional Excretion of Magnesium (FEMg) calculator is a crucial tool for assessing renal magnesium handling, particularly in patients with hypomagnesemia. In normal physiological conditions, the kidneys should conserve magnesium when plasma levels are low. This calculator helps clinicians determine whether magnesium losses are occurring through the kidneys or other routes.
In patients with hypomagnesemia:
An FEMg above 3-4% indicates renal magnesium wasting, suggesting that the kidneys are inappropriately excreting magnesium despite low serum levels.
An FEMg below 2% usually points to an extrarenal source of magnesium loss, typically gastrointestinal.
Research has shown that in hypomagnesemic patients, those with extrarenal causes of magnesium depletion had a mean FEMg of 1.4% (range 0.5-2.7%), while those with renal magnesium loss had a mean FEMg of 15% (range 4-48%).
