Blood work for potassium is pretty accurate.

It is the magnesium that becomes low. Serum tests only measure what is in the serum and not intracellular.

Typically most American's are somewhat low in magnesium.
Check what you eat, and look it up on that nutrition site I gave you.
http://www.nutritiondata.com/facts/v...roducts/2357/2

Recommended supplements for average people are 1/2 the RDA dose. For magnesium that would be about 200mg, elemental. Average RDA for magnesium is 350-400mg elemental for an adult.
See my magnesium thread for more details.

For potassium daily intake should be now around 4.5 grams or 4500 milligrams. (this was just raised from 3.5).

When a person is taking one or more drugs that affect QT and becomes dehydrated, that is a bad combination.
Mixing drugs is also dangerous.
Anna Nicole Smith's son Daniel died of this...and it was in the news for a long time. His was an accidental combination of methadone (not a high or toxic dose) with Lexapro SSRI.

When extreme even normal people with low magnesium can have an event. This most often happens with people on diuretics that are not supervised by doctors well...high dose diuretics for heart failure can kill people this way. The ER manages with IV magnesium sulfate. Doctors know how to monitor potassium, but they still remain rather clueless about magnesium.