49 C.F.R. § 655.61
Action when an employee has a verified positive drug test result or has a confirmed alcohol test result of 0.04 or greater, or refuses to submit to a test
(a)(1) Immediately after receiving notice from a medical review officer (MRO) or a consortium/third party administrator (C/TPA) that a covered employee has a verified positive drug test result, the employer shall require that the covered employee cease performing a safety-sensitive function.
(2) Immediately after receiving notice from a Breath Alcohol Technician (BAT) that a covered employee has a confirmed alcohol test result of 0.04 or greater, the employer shall require that the covered employee cease performing a safety-sensitive function.
(3) If a covered employee refuses to submit to a drug or alcohol test required by this part, the employer shall require that the covered employee cease performing a safety-sensitive function.
(b) Before allowing the covered employee to resume performing a safety-sensitive function, the employer shall ensure the employee meets the requirements of 49 CFR Part 40 for returning to duty, including taking a return to duty drug and/or alcohol test.