Most of time mercury won't show up on any test you do and some of the most mercury toxic and sickest patients are the ones with the lowest levels, because they are often retaining it and not eliminating it via the excretory organs and hair etc.

Some people in good health can show higher levels because they are often expelling any mercury they are absorbing.

Usually the most accurate way of testing for mercury, is a hair test, not to test for mercury levels (they were usually be very low), but to look at the mineral levels and usually in mercury poisoning, they are not as they should be, there is a pattern sometimes with mercury and certain mineral levels are deranged.

Sometimes, there is a method of provoking mercury into the urine/blood and getting a test with that, but even then, it can be inaccurate and has to be done properly to get a result.

I would never rely on any mercury test. The best way is testing the minerals, and checking your own symptoms also and sometimes taking something to mobilise/chelate mercury to see if symptoms are provoked and worsened, which is another indicator that mercury is causing issues.