49 CFR §655.61

1. (a)
   1. (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. (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. (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.
2. (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.