At what serum level of magnesium would the qt start to get longer? and what level considered risk of torsade de pointes?

Magnesium: I don't think this answer can be given with assuredness as we can only measure extracellular magnesium in clinical practice and we can't determine intracellular levels. All we can do is maintain normal levels of calcium and magnesium to avoid qt prolongation. In addition there are a number of medication interactions that we must be aware of when prescribing antiarrythmic medications. AskPharmacist