so I am taking this away from the above thread:

At any given time the body has certian elements in it, in order for the element to be detectable with current test methods, this element must be at a detectable level (duhh).
There are not known methods to destroy the element nor change it. Once it is introduced to the body it is on a steady decline as it is physicially purged from the body via urnation, unless more of the element is introduced.

so, to pass a drug piss test:
Do not introduce any more elements (quit smoking weed)
Piss alot (drink alot of water)

use home testing kits to check element levels
and if you do it for 78 days you will pass