Yeah I also wrote about nonexcretors and there are few people including autistic children which have genetic predispositions for the inability to detoxify. It takes much longer for metals to show and though it's not a guarantee, if you are toxic you may see the numbers going up higher even months or a year after you start.
Dr. Buttar states DMPS is more effective than DMSA. The method and dose however should be taken into consideration.