See my thread HERE on Sonnes #7. It always works for me. If you can't find any Sonnes or don't have the time or ability to order any online, just STAY CLEAN. Drink a lot of tea or water the night before and no more than a few glasses the morning of. Urinate as many times as you can before you get there and for safe measure, let the first bit go into the drain and not the cup.

I really think you will be fine. Your home test was probably set off by a false positive. Don't take any over the counter meds (like advil especially) except aspirin (aspirin has actually show to mess with some tests in the case of marijuana).