To be more specific ( and please excuse typos, i'm in a big hurry due to work) - the intracellular test is a red blood cell (RBC) test which measures the level of magnesium found inside the red blood cells NOTthe magneisum just floating in the blood serum. Symptoms of low levels of magnesium in red blood cells include heart palpitations, muscle cramps, muscle tremors and twitches and more.