One dose of DMSA wouldn't necessarily produce a reaction. You would need to use it systematically for a few days at least before you could get an idea of how you react.
I'll give you an example of what I mean. When I begin a round of chelating, I don't feel much of a reaction until the evening of the first day or the beginning of the second. If I
only went by my feeling after I took one dose, I would perhaps never know that I react at all.
Consider that a urine test only measures the mercury that comes of your body in your urine. The lack of mercury in your urine could be a sign that your body simply isn't eliminating mercury. This could be a sign of a very serious problem rather than a sign that there's no mercury.