The amount of magnesium in the P5P is quite small...it is listed as Oxide mix, and typically when this is done, it will be mostly oxide. Oxide is not absorbed.

However about 70% of people in US ARE low in magnesium and Nexium is a major obstacle for any being absorbed from food.
Most of the foods providing magnesium are not commonly consumed in US. So you may even need a quality mag supplement.
Blood work is only useful for very very low or high readings. This is because most magnesium is not in the serum, but in the cells. And the test you had most likely (unless a special lab did them) were serum tests.

This is my magnesium thread:
http://neurotalk.psychcentral.com/thread1138.html

Many people with PN can benefit from magnesium. You can get it from some special lotions, or by soaking in a tub with epsom salts as well. Some of us here us CVS Epsom lotion for the topical type.

Take your alpha racemic type on an empty stomach. 1/2 of it is the active form R...the other half is inactive. When it is gone
switch to the R form, preferably with the RALA stabilized form listed on the label. You will not need such high doses then.