What it is
When a child is hypomagnesaemic, the key question is where the magnesium is going. If the kidney is appropriately conserving, the loss is extrarenal (GI, poor intake). If the kidney is leaking it despite a low serum level, that’s renal magnesium wasting (tubulopathy, diuretics, cisplatin, aminoglycosides). The fractional excretion of magnesium (FEMg) measures exactly that fraction.
Formula
$$ FEMg,(%) = \frac{U_{Mg} \times P_{Cr}}{0.7 \times P_{Mg} \times U_{Cr}} \times 100 $$
The 0.7 factor is the load-bearing detail: roughly 30% of plasma magnesium is protein-bound and therefore not filtered at the glomerulus. Only the free ~70% is filterable, so plasma Mg in the denominator is multiplied by 0.7 to reflect what the tubules actually see. Omit it and FEMg is overestimated by ~40%.
When to use
A child with documented hypomagnesaemia and an unclear source. FEMg interprets cleanly only in that context — in a normomagnesaemic patient the cutoffs shift.
Worked example
Urine Mg 5 mg/dL, plasma Mg 1.2 mg/dL, urine Cr 40 mg/dL, plasma Cr 0.6 mg/dL:
$$ FEMg = \frac{5 \times 0.6}{0.7 \times 1.2 \times 40} \times 100 = \frac{3.0}{33.6} \times 100 \approx 8.9% $$
An FEMg of ~8.9% with a low serum Mg indicates inappropriate renal wasting — the kidney should be near-zero excreting if the loss were extrarenal.
Interpretation bands
- < 2% — appropriate retention / extrarenal loss (GI, intake).
- 2–4% — indeterminate; correlate clinically.
- > 4% — renal magnesium wasting.
In overt hypomagnesaemia, even FEMg > 2% has been reported as highly specific for renal wasting.
Pitfalls
- Don’t forget the 0.7 — it is the commonest FEMg error and inflates the result.
- Units must match (use the same units for uMg/pMg and uCr/pCr); the ratio is unitless.
- Recent IV magnesium or a magnesium-containing infusion spuriously raises urinary Mg — interpret off supplementation.
- A spot sample assumes steady state; AKI or fluctuating GFR degrades reliability.
Run it: Fractional Excretion of Magnesium (FEMg)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.