What do you think was your source of exposure to Hg and when do you think it ended?
This is important when considering testing. If it was a recent exposure in a chemical factory accident, you might still have elevated levels in blood. However if you have a problem with chronic exposure to heavy metals (HMs), blood is not likely the thing to test.
If you have plenty of symptoms of the effects of Hg from the neck down, you might still have enough Hg around to make a urine porphyrins or provoked urine test look suspicious.
The provoked urine test comes with risks. Some people react badly to the chelating agents used to kick up HMs prior to the urine collection.
People who are quite ill from Hg but did not have recent exposure from organic Hg (fish, vaccines, etc) probably won't have high levels in hair. From the 400+ hair tests I've viewed it is more likely that hair Hg will be rather low.
He also has more on testing for HM in his books--noamalgam.com
Cutler also has a theory about how HMs, especially Hg can foul up movement of essential elements into hair. However this is not going to provide certainty either, especially if exposure ended long ago or if you have found a helpful combo of supplements, diet and meds for your symptoms. There is a discussion of an early version of his theory in this old discussion.http://www.healing-arts.org/children/holmes.htm#results
Short of an autopsy, there probably is no test that will provide certainty. HMs hold on tightly to certain parts of the body, for example some hormone receptors and parts of the brain. If you mostly have symptoms related to those parts of the body, you might not have even suspicious test results.
In some of the Yahoo groups, the rule of thumb is that if you don't have any mercury in your teeth, you could do a trial round of chelation. Most people have a change in symptoms while taking them. This more commonly is a modest worsening of some symptoms, but sometimes there is improvement in some symptoms. For example a close friend sleeps better but is more lethargic and tired while on round.
Some times a followup hair test after a few months of chelation is more suspicious. That was my experience. My prechelation hair, urine and porphyrins tests were negative--more on that if requested.
Cutler has a chapter in is AI book on using symptom clusters as a diagnostic aid. I think this is a powerful tool.
Unfortunately your doctor may just be humoring you. Hopefully not, and you will get useful information from tests plus symptoms clusters to help you make a good decision.