Test Details: Magnesium blood test measures the amount of magnesium in your blood. Level that are too low are known as hypomagnesemia or magnesium deficiency. Levels that are too high are called hypomagnesemia. Symptoms of low magnesium include weakness, muscle cramps, twitching, confusion, irregular heart beat, seizures. Symptoms of high magnesium include muscle weakness, fatigue, nausea, vomiting, cardiac arrest etc.