(a) Each employer shall ensure that before a driver returns to duty requiring the performance of a safety-sensitive function after engaging in conduct prohibited by subpart B of this part concerning alcohol, the driver shall undergo a return-to-duty alcohol test with a result indicating an alcohol concentration of less than 0.02.
(b) Each employer shall ensure that before a driver returns to duty requiring the performance of a safety-sensitive function after engaging in conduct prohibited by subpart B of this part concerning controlled substances, the driver shall undergo a return-to-duty controlled substances test with a result indicating a verified negative result for controlled substances use.