When Mr. Hoo claims that the tea might be poisoned, Dr. Denton Deere diagnoses "paranoia".  His diagnosis in this case is rather obvious, as the doorman, Sandy McSouthers, who is "pacing the room" at the time of Deere's comment, points out.  McSouthers rejoins that "anyone who (is) not paranoid, after being told that the murderer would kill again, (is) really crazy" (Chapter 24).

Dr. Deere has the habit of offering unsolicited diagnoses throughout the narrative.  He does this to try to impress people with his medical knowledge.  The first time he sees Chris Theodorakis, he gives the opinion that the boy's disability stems from "pyramidal tract involvement", showing off his facility with complicated medical terms to get his fiancee Angela's notice.  Sometimes, Dr. Deere's pronouncements are entirely made-up, as when he observes Sydelle Pulaski's unusual limp and suggests "traveling sporadic myositis", even though he does "not have the least notion" about what is wrong with her (Chapter 5).

Dr. Deere's diagnoses at times are meaningless in their banality.  When Edgar Jennings Plum clears his throat before speaking to the game players, Deere whispers "nasal drip", to Chris's amusement (Chapter 8), and when the duplicity of Westing's plot begins to be revealed, Deere proclaims that "the man (is) insane" (Chapter 24).