The blood test that your doctor performed is called a SERUM magnesium test, which is part of any normal blood chemistry panel. Unfortunately, this test is not an accurate indicator of your body's storage of magnesium. Here's why: in order for your heart to beat (and for you to be alive) your body has to maintain a steady amount of magnesium in your blood at all times. It accomplishes this by pulling magnesium from your bones and tissues as needed -- including your muscles. So you can have a perfectly normal magnesium blood test and be LOW in magnesium in your muscles -- causing your cramps.