If unsure, the distinction between gastrointestinal losses and renal losses can be made by measuring the 24-hour urinary magnesium excretion. In addition, one can calculate the fractional excretion of magnesium (on a random urine specimen) with the following formula where U and P refer to the urine and plasma concentrations of magnesium (Mg) and creatinine (Cr). FEMg = [(UMg x PCr) / (PMg x UCr x 0.7)] x 100 If the fractional excretion of magnesium is above 2% in someone with normal renal function, the hypomagnesemia is likely secondary to renal magnesium wasting from drugs such as diuretics, aminoglycosides, or cisplatin. 2 grams of IV magnesium sulfate increased serum levels by a paltry median of 0.2 mg/dL. Rechecking too soon may give a false sense of security. If Magnesium level is 1.7 to 2, give 2 grams of MgSO4 IV. If Magnesium level is 1.3 to 1.7 give 4 grams of MgSO4 IV. Parenteral (IV or IM): Magnesium Sulfate (MgSO4) Magnesium Sulfate is 10% elemental (1 gram Magnesium per 100 ml solution) One gram MgSO4 contains 8.12 meq Magnesium One ml MgSO4 50% Solution = 4 meq Magnesium One ml MgSO4 10% Solution = 0.8 meq Magnesium Satyendra Dhar MD, @DharSaty #magnesium #hypomagnesemia #torsades
