I went through the exact same scenario for a prescreen test on a job a couple years back. Bought 3 of those home tests just to be sure, all of which were negative.

I decided to just piss and pray, turns out I should have been more concerned about the prescribed medication from my doc. He had me on cyclobenzaprine, a muscle relaxer, and darvocet, both of which came back positive on my test. No positive for MJ.

Had to have the doc fax some paperwork to the employer to verify the validity of the scrips I had, and all went well....

Until I figured out how shitty being a GM actually is. But that's another story for another thread.