E5.2

Differential Diagnosis Reasoning

Hold multiple possibilities, weight by evidence, narrow systematically. Don't commit prematurely.

Encounter card

Setting

During and after initial assessment; revisited as new information emerges.

Opening move

List the differential in order of probability and clinical significance. Hold each possibility until evidence rules in or out. Investigate distinguishing features actively rather than confirming the first guess.

Sample language

"(internal/written reasoning)"
"I want to be careful here — what you're describing could be a few different things. Let me ask some questions to sort it out."
"I'm thinking about depression, but also about thyroid issues and the medication change. Can we work through each?"

Listen for

Features that distinguish — onset, course, family history, medical context, response to prior treatment. Patient's framing of their experience.

Common pitfalls

Anchoring on the first diagnosis that fits. Confirmation bias — only looking for evidence that supports your initial impression. Missing medical contributors. Not asking the questions that would distinguish two competing diagnoses.

Red flags / escalate: Premature closure leading to wrong treatment direction. Missing bipolar in apparent unipolar depression. Missing medical mimic of psychiatric symptoms.

Documentation

Top diagnoses considered, distinguishing features, rule-outs in progress. "Differential: 1) MDD (recurrent), 2) Bipolar II depression — to rule out by hypomania history, 3) Hypothyroidism — TSH pending, 4) Adjustment disorder. Working diagnosis pending labs and collateral."

A wide initial differential narrowed by deliberate evidence is better than a single early commitment.

Warm grey-tinted clinical notebook page, deep ochre accent. An ordered differential — most likely → least likely, with don't-miss diagnoses kept on the list. Margin clusters on the ordering logic.

Differential diagnosis is the discipline of holding multiple possibilities open and narrowing them through deliberate evidence rather than premature closure. The most common diagnostic error in psychiatry is anchoring on the first plausible diagnosis and confirming rather than investigating it.

Open the differential wide initially. List the diagnoses you should consider before deciding which one fits best. Major depression, bipolar depression, adjustment disorder, anxious depression, substance-induced mood disorder, medical mimics (thyroid, B12, anemia, OSA), grief, demoralization, normal sadness. Wide is the default for the first pass.

Order by probability AND clinical significance. Major depression at the top because it's common; bipolar disorder kept on the list because missing it changes treatment; thyroid disease kept on the list because it's treatable and easy to test. "Don't-miss" diagnoses stay on the list even when probability is modest.

Distinguishing features shift the probabilities. What would rule in bipolar? Hypomanic history, antidepressant-induced switches, family history, mood instability. What would rule in thyroid? Symptoms beyond mood (cold intolerance, weight changes, energy, hair, menstrual). What would rule in substance-induced? Temporal relationship to substance use. Ask the questions that would distinguish; don't just confirm the first hypothesis.

Anchoring bias is the cognitive error to watch for. Once you've formed a hypothesis, you tend to weigh evidence supporting it more heavily and discount evidence against it. Counter this by deliberately asking: "What would make me wrong about this?" Investigate that question with the same energy you'd investigate the supporting evidence.

Reopen the differential at decision points: when treatment fails, when symptoms change, when new information emerges. The "treatment-resistant depression" patient often turns out to be a missed bipolar disorder, or a thyroid problem, or substance use, or an inadequate trial. Failure to respond is a signal to reconsider, not to escalate within the same framework.

Distinguishing features — what would shift the probability between two competing diagnoses. Sample example. Margin notes on the questions to ask.

The anchor

Differential diagnosis is held wide initially, narrowed by deliberate evidence. Avoid premature closure; investigate distinguishing features rather than confirming the first impression.

The anchoring bias — premature closure on the first plausible diagnosis. Margin clusters on how to counter it.

Prove it

A patient presents with depression that hasn't responded to two SSRIs. What differential considerations should you actively investigate?

This connects to

E5.1

Biopsychosocial Formulation

The integrative parallel.

E5.4

DSM vs Dimensional vs Functional

Frameworks for the differential.

Locked concepts unlock as you reach them on the path.