I had a urine challenge test using DMSA, too, but not an IV test.
I think unless you're completely "clean" of supplements for a long time, urine metal tests have little value. For example, when pressed my doctor couldn't really say if I excreted all that copper and zinc because I actually had high systemic levels of these minerals, or because I'd been supplementing with them. She couldn't really advise me whether I should stop supplementing with them or not.
IMHO it's a pretty safe bet all of us need to chelate at least mercury and lead, just because of the world we live in. The metals we do need that get chelated (magnesium, calcium, etc.) along with the bad stuff can be replaced with supplements. I'll never do another urine metals test (might try a hair test in the future) but will always chelate occasionally, just to be safe. Chelating put me much farther down the path to wellness.
Just my $0.02. YMMV.