Calculates fractional magnesium excretion using paired serum and urine magnesium and creatinine values. This premium guide explains physiology, calculation, interpretation, limitations and evidence.
Overview
Calculates fractional magnesium excretion using paired serum and urine magnesium and creatinine values.
Clinical Significance
Can help distinguish renal magnesium wasting from reduced intake or gastrointestinal loss.
When to Use
- Investigate unexplained hypomagnesaemia.
- Use paired samples before major replacement when practical.
- Interpret with kidney function and medicines.
How It Is Calculated
- FEMg = (urine Mg × plasma creatinine) ÷ (0.7 × plasma Mg × urine creatinine) × 100.
- MedicalC validates required values and rounds only the displayed result.
Interpretation
- Can help distinguish renal magnesium wasting from reduced intake or gastrointestinal loss.
- Review source measurements, collection quality and clinical trajectory as well as the final value.
Worked Example
Enter clinically plausible values in the displayed units. MedicalC applies: FEMg = (urine Mg × plasma creatinine) ÷ (0.7 × plasma Mg × urine creatinine) × 100. Verify the result against the source data before use.
Patient Considerations
- Consider kidney function, diuretics, intravenous fluids, nutrition, body composition and rapidly changing physiology.
- For timed urine calculations, confirm collection duration, volume and completeness.
- For electrolyte prescriptions, include all oral, enteral, intravenous and medication sources.
Limitations
- The 0.7 filterable-fraction factor is an approximation.
- Reduced GFR alters expected values.
- Recent magnesium treatment can mislead.
Clinical Pearls
- Use paired serum and urine values obtained as close together as possible when the formula requires them.
- A derived renal index is only as reliable as the collection and assays used.
- Check whether treatment occurred between sampling and interpretation.
Common Mistakes
- Mixing µmol/L and mmol/L creatinine values.
- Using spot concentrations as daily excretion rates.
- Ignoring incomplete timed urine collection.
- Treating a maintenance estimate as a replacement or emergency-correction regimen.
- Using a derived acid–base value without checking albumin, lactate and the blood gas.
Evidence Base
Principal source: Elisaf M, et al. Fractional excretion of magnesium in normal subjects and patients with hypomagnesemia. Magnes Res. 1997.
Frequently Asked Questions
What does the Fractional Excretion of Magnesium calculator do?
Calculates fractional magnesium excretion using paired serum and urine magnesium and creatinine values.
Can this result establish a diagnosis?
No. It is an adjunct that must be combined with clinical assessment and current guidance.
Why are timing and units important?
Derived results can be misleading when values are non-contemporaneous or use incompatible units.
When should the calculation be repeated?
Repeat when treatment, kidney function or the relevant measured values change and a new result could influence care.
Does this replace a specialist protocol?
No. Direct measurement, laboratory interpretation and specialist protocols take priority.
References
- Elisaf M, et al. Fractional excretion of magnesium in normal subjects and patients with hypomagnesemia. Magnes Res. 1997. — Primary publication, guideline or authoritative specialty source.
Reviewed by: MedicalC Clinical Editorial Team
Last reviewed: July 2026