Blood serum levels are not the end all of nutrient testing.
Test ranges were done by taking people at random with no symptoms,
and testing their levels of things. There is no real accuracy with that,
and it is just a statistical exercise, and many ranges are very very old, besides and only test serum levels.

There are intracellular labs, that test the red blood cell content.

Spectracell is one.
http://www.spectracell.com/

Alternative and integrative doctors use other labs like this to
perform more accurate testing.

Magnesium in particular is very inaccurate in the mid ranges.
When very very low (like in chemo patients or those on high dose long term diuretics), it is a medical emergency...because of the heart. When very very high, due to poisoning from enemas, or abuse of laxatives, or kidney failure, it is also a medical emergency. But in between, there is no guarantee that a "normal" reading is doing anything for you. Magnesium in the serum is useless because it is not working the many systems and cells of the body. The mitochondria use magnesium, the nervous system, metabolism, neurotransmitter synthesis ..etc all happen inside cells.

B12 tests are not accurate...because doctors don't understand the ranges...either. Low B12 is very common in US because our labs continue (after a decade now) to report numbers below 400 as "normal". Same with Vit D.

If you come to PN forum and read Sallysblooms' posts, she goes to an integrative doctor who is reversing her severe autonomic neuropathic issues. Look her up in members list, and read her posts. She is quite interesting.