Testing for mercury is not very useful. Mercury accumulates in the liver, kidneys, brain, nervous system, thyroid and other organs. The amount of mercury in the blood, or the amount of mercury excreted won't tell you anything about how much mercury is in the liver, kidneys and brain that is causing problems. Mercury toxicity should be diagnosed based on symptoms. Some doctors like to do a DMSA challenge test, where a large dose of DMSA is taken, then mercury excretion after that is compared to mercury excretion before that. The test gives little information though, and some people get very sick from it.

http://www.flcv.com/cfsfm.html