Blood magnesium is low

Anyone else have this? It’s strange because i drink a lot of electrolytes for POTS and i also take magnesium citrate regularly for years. it’s the only abnormality in my many blood tests and scans over the years. Doctor told me to take 450mg of magnesium daily but I already take that much if not more. From research it seems blood magnesium test is not a good indicator of magnesium levels in the body so i’m really lost.