Fractional Excretion of Magnesium (FEMg) Calculator
Fractional Excretion of Magnesium (FEMg) Calculator
Identify renal magnesium wasting as the cause of hypomagnesaemia, correcting for the fraction of serum magnesium that is protein-bound and never filtered.
Fractional Excretion of Magnesium (FEMg)
Mg clearance ratio (× 0.7)Urine Mg 6 mg/dL, serum Mg 1.4 mg/dL, urine creatinine 90 mg/dL, serum creatinine 2.4 mg/dL
Formula
- 0.7
- correction factor — only about 70% of serum magnesium is free and protein-unbound, and only the free fraction is filtered at the glomerulus. Omitting this factor overstates true filtered magnesium and produces a falsely low result.
- urine Mg, serum Mg
- mg/dL
- urine creatinine, serum creatinine
- mg/dL
Worked example
Urine Mg 6 mg/dL, serum Mg 1.4 mg/dL, urine creatinine 90 mg/dL, serum creatinine 2.4 mg/dL
6 × 2.4 = 14.4
0.7 × 1.4 × 90 = 88.2
14.4 ÷ 88.2 × 100 = 16.3%
With hypomagnesaemia (1.4 mg/dL) this is well above 4% → renal magnesium wasting
Interpretation in hypomagnesaemia
| FEMg | Pattern | Consider |
|---|---|---|
| < 2% | Extrarenal loss | Diarrhoea, malabsorption, laxative use, poor intake |
| 2 – 4% | Borderline | Correlate with the clinical picture |
| > 4% | Renal magnesium wasting | Diuretics, calcineurin inhibitors, cisplatin, PPIs, tubulopathy |
Why the 0.7 correction matters
Roughly 70% of circulating magnesium is free and ultrafilterable at the glomerulus; the remaining 30% is bound to albumin and other plasma proteins and is not filtered at all. Every other fractional excretion index on this site treats the whole serum concentration as filterable, because sodium, potassium, urea and phosphate are not meaningfully protein-bound. Magnesium is the exception, and the formula corrects for it by multiplying serum magnesium by 0.7 in the denominator before comparing it with urine magnesium.
Leaving the 0.7 out — using the same formula shape as FENa — systematically understates FEMg, because it divides by a serum magnesium figure that is too high relative to what was actually available to filter. In a borderline case this can be enough to shift a result across the 2% or 4% threshold and change the interpretation, so the correction is not a minor refinement; it is what makes the number meaningful.
As with the other tubular indices, FEMg is read against the paired serum magnesium. In a magnesium-replete patient the tubules reabsorb magnesium efficiently in the thick ascending limb and distal tubule, and FEMg sits in the low single digits. In hypomagnesaemia the expected compensatory response is even tighter conservation, so a value above about 2%, and certainly above 4%, means the kidney is losing magnesium it should be holding onto.
Frequently asked questions
Why does the FEMg formula include a 0.7 factor?
Because only about 70% of serum magnesium is free and unbound to protein, and only that free fraction is filtered at the glomerulus. The 0.7 corrects the denominator so it reflects filterable, not total, serum magnesium.
What happens if the 0.7 factor is left out?
The result is systematically too low, since it divides by a serum magnesium value that includes the protein-bound, unfilterable fraction. This can shift a borderline result across the diagnostic threshold.
What FEMg suggests renal magnesium wasting?
In a hypomagnesaemic patient, above about 4% indicates the kidney is inappropriately wasting magnesium — common causes include diuretics, calcineurin inhibitors, cisplatin, aminoglycosides and proton pump inhibitors.
What FEMg suggests extrarenal loss?
Below about 2% in a hypomagnesaemic patient shows the kidney is conserving magnesium appropriately, pointing to a gastrointestinal or dietary cause instead.
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References
- Elisaf M, Panteli K, Theodorou J, Siamopoulos KC. Fractional excretion of magnesium in normal subjects and patients with hypomagnesemia. Magnes Res. 1997;10(4):315–20.
- Van Laecke S. Hypomagnesemia and hypermagnesemia. Acta Clin Belg. 2019;74(1):41–7.