Drug Screens are different from drug tests. Screens are done instantly and just do a basic check. Additives and whotnot can be used to pass.
A lab actually checks the contents of the urine itself. Most labs check really good for additives or dilution and so on. It's ususally a lot easier to pass a drug screen, than a drug test. If you passed a screen without cheating, then you should pass the test to.
Don't hold me to it though if you fail the test.
Good luck!