Differential Diagnosis Basics

1. What a Differential Actually Is

Differential comes from "differentiate" — to tell things apart. So a
differential is a list of the things you're telling apart, not a single
guess.

One chief complaint can come from a lot of different problems. Chest pain
could be cardiac. It could be a pulled muscle. It could be a pulmonary
embolism. A differential is the short list that holds several of those
possibilities open at once, instead of collapsing to one cause the second
you walk in the door.

It's built from the same inputs as your field impression — chief complaint,
history, exam findings. But it's framed differently. A field impression is
your one working label. A differential is the set of candidates you pulled
that label from, and you keep more than one of them alive until the
evidence says otherwise.

You don't have to narrow it to exactly one entry before you can act. You
act based on the most dangerous entry still on the list.

So think of the differential as a running roster, not a single suspect.
Entries move up and down that roster as findings come in. None of them get
struck off just because a better story showed up. That's what keeps you
from locking onto the first plausible cause and missing the one that
actually kills the patient.

The exam fact: a question that asks what else should be considered, or
what's "most concerning" given a presentation, is testing the differential
as a set. The correct answer names the dangerous entry. Not just the
probable one.