Dr. Christopher Shade is, I believe, the world’s foremost authority on mercury detoxification. In this video he discusses the mechanics of the DMSA “challenge” test for mercury toxicity and its complications.
In cases of chronic mercury toxicity it is known that the kidney function is often compromised. Chelating agents such as DMSA and DMPS are meant to mobilize toxic metals through already compromised kidneys. The inflammation resulting from mobilizing these metals makes the process of detoxification that much more damaging and inhibits the whole process.
Quicksilver Scientific has developed methods of testing that do not rely exclusively on the already-stressed kidneys but instead use the body’s inherent detoxification pathways to allow mercury and other heavy metals to be extracted from the body.