Fractional Excretion of Magnesium

Used to distinguish renal vs. extrarenal causes of hypomagnesemia and to detect tubular injury. A FE Mg <2%** suggests appropriate renal conservation (extrarenal loss), while **>4% indicates renal magnesium wasting (genetic tubulopathies, diuretics, tubular injury) 1 [Level III evidence]; values ~15% strongly support renal loss 2 [Level II evidence]. Also useful for aminoglycoside nephrotoxicity detection 3 [Level II evidence].

Helps distinguish renal Mg wasting from extrarenal loss: FEMg >3–4% in normal GFR suggests renal wasting; <2% suggests extrarenal causes. Use cautiously because FEMg rises with declining GFR and proteinuria‑related tubular injury increases FEMg independent of true wasting 1. In CKD or diabetes, elevations may reflect tubulointerstitial damage, not Mg deficiency. Unreliable during diuretic use, acute illness, or aminoglycoside exposure. Avoid over‑interpreting single measurements.