Unfortunately, blood tests for magnesium are not really an accurate way to test. My primary doctor admitted that to me when I asked him about it. The reason is, is that 99% of magnesium is found inside of the cells while only 1% is found in the blood. Potassium is the same thing. Instead, the body gives symptoms of magnesium shortage. Among them are tight neck, shoulder and jaw muscles, less than 2 bowel movements per day, easily startled by loud noises, lack of good sleep, anxiety, unfounded fears and apprehensions, depression, heart palpitations, skipped beats etc. I learned this from a world class nutrition researcher and formulator of nutrients. He has spent 38 years in research of nutrition and has trained around the world with some of the best researchers ever. He does market a line of nutrients which have changed my life (and my wife). She was anxious, depressed, hopelessness etc. and when I put her on a superior magnesium, these symptoms improved 90% or better! But most magnesium will not get these results. What we use is called Mag Plus and we order it mail order at: 1-877-222-9235. It can change your life. And no, I don't sell a thing. Just want to pass on something that is head and shoulders above what you usually find on the shelves.