I have had various blood tests for many years, all being in normal range. Mercury is a neuro-toxin: it likes electric bundles like the brain. I don't think blood or urine samples give full picture of mercury toxicity, for many reasons. Mercury gathers in our organs, so isn't really in the urine or blood, at least not at super-toxic levels. And, as humans vary in sensitiity, what would be a toxic-level for one person, may not be for another. There are so many variables. But, if a person has exposure to mercury (in whatever form) and has the symptoms of poisoning, it seems a reasonable conclusion to me to at least explore the possibility of systematic toxicity.