Pursuant to the ADA, when dealing with an employee, an employer may make disability-related inquiries and require medical examinations only if they are job-related and consistent with business necessity. The ADA makes it clear that these restrictions apply to all employees, not just those with disabilities.
The first issue that must be addressed is whether the employer's question is a disability-related inquiry or whether the test or procedure it is requiring is a medical examination. It must then be determined whether the person being questioned or asked to submit to a medical examination is an employee. If the person is an employee (rather than an applicant or a person who has received a conditional job offer), the final issue is whether the inquiry or examination is job-related and consistent with business necessity or is otherwise permitted by the ADA.
Where the first two requirements are satisfied, it must be determined if the inquiry or examination is job-related and consistent with business necessity. According to EEOC enforcement guidance, an employer's reasonable belief that an employee's ability to perform essential job functions will be impaired by a medical condition or that he will pose a direct threat due to a medical condition must be based on objective evidence obtained, or reasonably available to the employer, prior to making a disability-related inquiry or requiring a medical examination. Such a belief requires an assessment of the employee and his position. It cannot be based on general assumptions.
To fall under this exception to the general prohibition against medical examinations, "an employer must show that the asserted business necessity is 'vital to the business,' that the examination 'genuinely served the asserted business necessity,' and that 'the request is no broader or more intrusive than necessary' (Shannon v. Verizon New York Inc., No. 1:05-CV-0555, 2009 WL 1514478, at *3 [N.D.N.Y. May 29, 2009]).
As one can see, this can be an involved process. It often makes sense to consult with an attorney prior to insisting an employee undergo a medical exam. Such a consultation can help the employer determine if an examination is permitted and also avoid future allegations of discrimination.