Fractional excretion of magnesium (FEMg)
Fractional excretion of magnesium: FEMg = (urine Mg x plasma Cr) / (0.7 x plasma Mg x urine Cr) x 100; above ~2% (Elisaf cutoff ~4%) in hypomagnesemia indicates renal magnesium wasting.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Urine magnesium
- 2.0 mg/dL
- Plasma magnesium
- 1.2 mg/dL
- Urine creatinine
- 50 mg/dL
- Plasma creatinine
- 1.0 mg/dL
Result: FEMg 4.8%: above ~2%, consistent with renal magnesium wasting; the Elisaf series saw renal loss from 4% upward.
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What the result means
- Reading
- in hypomagnesemia, above ~2% suggests renal magnesium wasting and lower values point to an extra-renal cause.
- What the source saw
- Extra-renal hypomagnesemia ran 0.5-2.7%, renal loss ran 4-48%, controls averaged 1.8%. Between 2.7 and 4% lay in neither observed group.
What you enter
- Urine magnesium
- Plasma magnesium
- Urine creatinine
- Plasma creatinine
How this is calculated
- Formula
- FEMg (%) = (urine Mg x plasma Cr) / (0.7 x plasma Mg x urine Cr) x 100; the 0.7 corrects for the protein-bound, non-filterable plasma fraction.
Elisaf M, Panteli K, Theodorou J, Siamopoulos KC. Fractional excretion of magnesium in normal subjects and in patients with hypomagnesemia. Magnes Res. 1997;10(4):315-320. Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.