I didnt work for Labcorp, But I did work for Quest Diagnostics. My pre-employment required a blood and urine test, I was allowed to pee in privacy. The blood was for testing heavy narcotics like Vicodin and the like.

Since I work in the Tox area, I do know that you have to pee a certain amount in order for the test to be run. If you don't pee that amount whomever will be notified and they will require them to send more specimen. Thus giving you more time to detox ( lack of a better word sorry)

Hope this sort of helps