A zillion years of medical and graduate school, and nearly three years of residency, and I still don't understand what it means when people say that such-and-such is a diagnosis of exclusion. Any diagnosis is a diagnosis of exclusion!
Often, I think, people use that phrase as a retelling of the widespread myth that psychopathologies can't be diagnosed through testing. ("Anxiety is a diagnosis of exclusion.") Or a claim that only laboratory tests or imaging matter, not the history or physical. ("Hepatorenal syndrome is a diagnosis of exclusion.")
The next time someone calls a diagnosis a D of E, by gum, I'm going to ask them just what they mean.