Differential diagnosis: from findings to conclusions

Notions clés

Consider the common mimics first

Before settling on a primary mental disorder, the psychologist asks whether symptoms are feigned, caused by a substance or medication, or caused by another medical condition; most DSM-5-TR criteria sets exclude symptoms better attributed to a substance or medical condition.

Timing and course

When symptoms began relative to substance use, illness, stressors or developmental stage, and whether they come in episodes or persist, often separates competing explanations more clearly than a count of symptoms does.

Substance-induced versus independent

DSM-5-TR points to an independent psychotic disorder when symptoms began before the substance use or persist for a substantial period, about a month, after intoxication or withdrawal ends. In a Danish registry, about a third of people with substance-induced psychosis later received a schizophrenia or bipolar diagnosis.

More than one diagnosis

Co-occurring disorders are common, so the aim is to account for every important finding, recording each diagnosis whose criteria are met, rather than forcing all symptoms into one label.

Recommendations from findings

Useful recommendations address the referral source's concerns and follow from specific findings, and they say what further evaluation would resolve any uncertainty that remains.

Exemple commenté

Selim, 22, is brought to a clinic after a week of believing that people in passing cars are following him and that his phone has been hacked to track him. He has used methamphetamine heavily for a month and last used four days ago. He has no psychiatric history, and an uncle has schizophrenia.

1. The psychologist first arranges a medical evaluation to look for intoxication effects, delirium and other medical causes.
2. Because the beliefs began during heavy stimulant use and there is no earlier psychotic history, a substance-induced psychotic disorder is the leading hypothesis, with a schizophrenia spectrum disorder kept on the list given his family history.
3. Course will decide: if the beliefs persist for about a month after use stops, an independent psychotic disorder becomes more likely.
4. Recommendations follow: treatment for stimulant use, reassessment of the psychotic symptoms after about a month of abstinence, and longer follow-up, since substance-induced psychosis often precedes a later diagnosis of schizophrenia or bipolar disorder.

Exclude substance and medical causes, then let the time course do much of the diagnostic work.