Tests for magnesium in the serum are not really reliable in the "normal" range. They are only good medically for very very low, or very high values to indicate severe deficiency, or poisoning.

The mid range values do not really relate much to functions that can be measured. You can look this up on the net...as many magnesium experts, seem to agree with this explanation.

For therapeutic use medically extremely high doses are given...for chemo patients and migraine treatments. (often IV forms up to 6 grams a bag.)

That does not mean people should self medicate at those high doses, but it does illustrate that magnesium testing is odd in many ways.