The typical magnesium
test is actually an Intensive Therapy Unit
test, in a small, tight range, meant to keep you alive. That is according to Dr. MyHill.
The RBC magnesium test is said to be better, but it can still miss deficiencies. The best are the loading or tolerance test, but this can be hard to get. (See article by Michael Schachter M.D.)
Basically, magnesium deficiency is very common these days, and the deficiency symptoms are very obvious when researched.