A patient got $$ credit placed in their account by just calling about an illness without documentation required.

However, a letter documenting inability to travel for medical reasons was required to use the credit some months later.

Since the patient never informed us about the illness, I could not write the letter. Thus the money was lost.

So it might be prudent to let your clinician know, even just a email, that you are cancelling a flight for medical reasons even if DL does not ask up front.