All the material I've researched says approximately the same thing...

Mercury stays in the bood for a very short period of time (minutes). It's quickly removed and absorbed by various organs. Therefore, blood tests which search for mercury are virtually useless.

There are, however, combinations of tests that can be done including blood porphyrins which (in combination with other tests) can indicate the presence of mercury. The vast majority of doctors do not know this (I grew up in a medical family?many doctors).

If you're reading It's All In Your Head by Hal Huggins, there's a complete section in there on this subject.