Reasonable suspicion drug testing is performed when a supervisor suspects that an employee may actively using drugs at the workplace. To determine reasonable suspicion a supervisor should have evidence or reasonble cause. An example would be direct observation by a supervisor or another employee, physical evidence, or a pattern of unpridictable behavior, confusion with the inability to complete normal tasks.

Reasonable suspicion drug testing requires special training for supervisors in order to be effective to protect the company from bias, due to its discretionary nature.
Employers with safety sensitive jobs subject to federally mandated laws have guidelines of suspicious behavior and are given specific steps to perform to ensure consistent and non-bias testing.
Employees who are tested under reasonable suspicion are usually not allowed to return to work until the test results are determined.