Q: What is FeMg?

Answer:  Fractional excretion of magnesium

In refractory hypomagnesemia, it may require to find the route of continous loss of magnesium from the body. Two major routes of elimination of magnesium from the body are either gastrointestinal (GI) and renal. To distinguish magnesium loss between these two routes can be made by FeMg by formula

                           UMg  x  PCr
 FEMg    =    —————————   x    100 percent
                    (0.7  x  PMg)  x  UCr


U = Urine
P = Plasma
Cr = Creatinine


FEMg more than 2% indicates inappropriate renal wasting.


References:

1. Topf, J.M. & Murray, P.T. (2003) Hypomagnesemia and hypermagnesemia. Reviews in Endocrine & Metabolic Disorders, 4, 195– 206.

2. Elisaf M, Panteli K, Theodorou J, Siamopoulos KC. Fractional excretion of magnesium in normal subjects and in patients with hypomagnesemia. Magnes Res 1997; 10:315.