That might mean you did not have recent exposure but not tell you much about old exposures. It might suggest that Hms are not the problem. However check out Boyd Haley, former chair of the chem dept at the U of KY on this issue. He says that those who are very sick from Hg have levels of it in hair and nail clipping tests than healthy people. https://www.youtube.com/results?search_query=boyd+haley+%2B+alzhimers
He explains that this is a retention toxicity. People that don't get rid of it at a normal rate will have problematic levels in parts of the body that hold on rather tightly---brain, thyroid, adrenals, certain hormone receptors, bone (lead), etc. We don't have good ways to test those levels--short of an autopsy!
Therefore we need to look for the effects of HM toxicity, including symptom clusters. A. Cutler has a chapter in his book, Amalgam Illness on that.
Urine porphrins is an example of a test that looks for the effects of HMs, but like most tests, it is not as good for exposures that ended a long time ago, especially in people who are now eating right and taking antioxidants.