14 CFR Part 382 Nondiscrimination on the Basis of Disability in Air Travel (Air Carrier Access Act) (with amendments issued through May 2016)

(d) As a carrier, you may require that a passenger with a medical certificate undergo additional medical review by you if there is a legitimate medical reason for believing that there has been a significant adverse change in the passenger's condition since the issuance of the medical certificate or that the certificate significantly understates the passenger's risk to the health of other persons on the flight. If the results of this medical review demonstrate that the passenger, notwithstanding the medical certificate, is likely to be unable to complete the flight without requiring extraordinary medical assistance (e.g., the passenger has apparent significant difficulty in breathing, appears to be in substantial pain, etc.) or would pose a direct threat to the health or safety of other persons on the flight, you may take an action otherwise prohibited under §382.21(a) of this part.