Differential Diagnosis and Case Formulation

Why symptom matching fails

Most symptoms appear in most disorders. Poor sleep, low mood, and difficulty concentrating are compatible with a depressive disorder, an anxiety disorder, a substance-induced picture, a trauma-related presentation, hypothyroidism, sleep apnea, or ordinary sleep deprivation. Matching a symptom to the first disorder it suggests is not diagnosis.

What distinguishes explanations is pattern, timeline, course, and confounders. When did it start, relative to what? Does it persist when the suspected cause is absent? Does the course fit? What else could produce exactly this?

The method: GATHER, GENERATE, TEST, RESOLVE

GATHER the evidence: onset, duration, course, severity, impairment, medical history, medication, substance use, developmental stage, trauma, and family history.

GENERATE a genuine differential. If three of your candidates are implausible, you have not built one. Include at least one medical and one substance-related possibility whenever the presentation could support them.

TEST each candidate against the evidence. For every one, ask what supports it, what argues against it, and what you would need to know to rule it in or out. That third question is the one candidates skip.

RESOLVE, or decide that you cannot yet. "Insufficient information to distinguish, here is what I will gather" is a legitimate and often correct formulation. Forcing a premature diagnosis into the record is worse than naming the uncertainty.

The substance question

When symptoms and substance use coexist, the deciding evidence is temporal. Symptoms that predate the substance use, or that persist through a sustained period without it, support an independent disorder. Symptoms that appear only in the context of use, or that track intoxication and withdrawal, support a substance-induced picture.

Without a history of an abstinent period, you frequently cannot distinguish them, and saying so is the accurate answer.

Common mistakes

Treating the exclusion of one cause as confirmation of another. A normal cardiac workup rules out a cardiac cause and says nothing about anything else.

Anchoring on the referral question and never seriously considering alternatives.

Missing medical contributors, particularly thyroid disorders, sleep disorders, medication effects, and, in older adults, reversible causes of cognitive change.

Reading any reported voice as psychosis. Hearing a deceased spouse call your name while falling asleep is a recognized grief phenomenon.

Diagnosing adjustment disorder when the presentation meets criteria for a specific disorder.