First of all if you took a test and a line shows up undiluted or not its a fail, Ive tried the at home piss tests, and almost all of them say any sign of a line is a fail all the time.

Now Back to the diluted test that you passed if you can mimic that in your real exam thats bulletproof. Having some color to the urine makes a lot of difference. Again though i would refer you to N2's dilution post at the top of the Drug testing thread as it is very thorough and i would follow it just to give you some confidence.

One other test it could be is a saliva mouth swab as those are quick and easy and they can do them on the job site, I just took one and am awaiting the results. If thats the case FakeBoobsRule says it only can go back roughly 96 hours. I just brushed the hell out of my mouth and rinsed with Listerine 3 times a day.