The key to a good d ifferential diagnosis is to start with a broad , all-inclusive
differential based on the major points of the case and narrow it down logically
to a smaller, "working" differential based on the specifics of the case. If the patient's
signs/symptoms become atypica l , or new information becomes ava ilable,
you can go back to the broad differential; in this way, you will not miss
u nusual presentations of disease . Consider a 50-year-old man with hypertension,
diabetes, and hyperlipidemia presenti ng with stroke: the expanded differential
should i nclude all causes of stroke, but the working d ifferential features
atheroth rombotic disease, hemorrhage, a nd embolism as l i kely etiologies. If
the CT scan shows a mass lesion, then you'd better return to the broad differential
to include tumor and brain abscess. If the patient develops a fever and is
found to have a sed i mentation rate of 1 00, then you should return to the expanded
differential and retrieve vasculitis and infection to add to your working
differential.
Without a complete initial differential diagnosis, or a return to .the initial differential
when a typical case becomes atypical, you will miss diagnoses.