5 C.F.R. § 339.302
An agency may, at its option, offer a medical examination (including a psychiatric examination or psychological assessment) in situations where the agency needs additional medical documentation to make an informed management decision. This may include situations where an employee requests, for medical reasons, a change in duty status, assignment, working conditions, or any other different treatment (including reasonable accommodation or return to work on the basis of full or partial recovery from a medical condition) or where the employee has a performance or conduct problem that may require agency action. Reasons for offering an examination must be documented. When an offer of an examination has been made by an agency and the offer has been accepted by the applicant or employee, the examination must be carried out in accordance with the authorities cited in § 339.103 . The results of the examination must also be used in accordance with the authorities cited in § 339.103 .
5 C.F.R. §339.302