A challenge test isn't worthwhile though. It won't tell how poisoned your body is. the only thing it might tell is how effective that chelating agent is in your body at that specific dosage in removing mercury. If it is at a very high dosage though, then it might not even be indicative of how well that chelating agent works in moderate doses. The amount of mercury excreted in a challenge test is not in proportion to the amount in the body.