Lets talk about a common case of vanishing Magnesium – A patient admitted with dehydration & hypokalemia had serum Magnesium levels at 1.2, after an IV Magnesium Sulfate bolus (2 grams over 4 hours), levels were rechecked 6 hours later , came back as 2.0, next morning it was back to to 1.4 . Where did all that Magnesium vanish !! Serum Magnesium levels, like most humans, do not like to be pushed hard- An IV Magnesium bolus can raise serum Mag levels pretty good over a few hours – but is a temporary affair. Because , the rapid bump in Mag levels makes kidneys flush out the excess and whats left slowly seep into tissues, especially bones , until a new equilibrium between blood and tissues is achieved.
So the point of this short & sweet post is this – after IV Magnesium infusion as a bolus, a level recheck done too soon can give you a false sense of success due a temporary bump. The slower the infusion, the more sustained the level raise will be. Raising levels up fast urgently and rechecking to see if goal levels are maintained can be important for situations like Torsades or other arrhythmias with very low Mag levels, Eclampsia, etc. – but in other less urgent situations, unlike potassium levels , Magnesium levels should ideally be checked 24-36 hours after the renal-flush and body-equilibrium is done , to give you a more true picture.
I usually do a IV 2gm to 4 gm slow push over 4 to 6 hours and start oral Magnesium supplement 2-3 times a day the same time, recheck levels on day 3.
