Fractional Excretion of Magnesium

Urine specimen: spot

fractional excretion of magnesium =

= ((serum creatinine) * (urine magnesium)) / (0.7 * (urine creatinine) * (serum magnesium)) * 100%

where:

• 0.7 reflects the nonalbumin-bound fraction.

• The units should cancel as long as the same type of units are used.

Interpretation:

• A fractional excretion of magnesium > 4% indicates renal magnesium wasting.

Limitations:

• Due to variation in magnesium excretion the determination of 24-hour urine magnesium concentration is considered more accurate (if the collection is accurate).