The Office of Drug and Alcohol Policy and Compliance, with the Office of General Counsel, issued 49 CFR Part 40 Question and Answers. The Q&As explain:
1.) When may a collector give an employee permission to leave a collection site
2.) What happens if an employee leaves the collection site prior to the completion of the testing process
To view all Q&As: http://www.dot.gov/odapc/part40QandA