CDI stands for Certified Deaf Interpreter. This person is one who is deaf themselves, or hard of hearing, and has been certified to be an interpreter by the Registry of Interpreters for the Deaf. You may need to consider having someone like this available when dealing with a person who’s communication methods are quite unique. Someone who can hear well may not be able to fully grasp what your client/patient is trying to tell you. This is where the CDI will come in.