False positives won't matter. Once they send it to GC/MS they'll know exactly what the substance is.
For businesses that do this, I recommend not peeing. Just stall. By law they have to give you 3 hours to sit there and pee. So just sit there. You're being paid anyway. So sit and wait for them to release you. They'll be shoveling water down you. so you'll have to try to hold it. or if your lucky and there's aplant somewhere there. just pour the water in the planter when nobodies watching. Fucking with the boss is always a good thing.