Here's what I would recommend doing. Take a B vitamin a few hours in advance. Begin drinking water 24 hours before. Piss right before you leave. Here's why:

Begin drinking water something like 24 hours before the test. Your urine should be clear - this is what you want. Clear urine means less metabolites per unit.

If you are drinking enough water you should take a piss right before you leave for the test. This will clear out any metabolites that have been collecting. Believe me, I have been tested before. The nervousness alone makes me piss like a racehorse. (For the record, I always passed:jointsmile

Now they probably wouldn't like to see clear urine, so I recommend buying a small bottle of B complex. Since your metabolism seems oddly slow, I would say take one the day before. Jot down how long it takes for your urine to yellow. In fact, yellow is an understatement, it makes your piss NEON if you're not full of water. Take it before the test.

If you want to go in with full confidence, try the water/pre-piss/B vitamin beforehand with a home test. If you pass the home test, you are good.

Good luck!
-Fnatic.