What is the most consistent diagnostic feature of delusional disorder?

Daily Functioning is Not Markedly Impaired in Delusional Disorder

The most consistent diagnostic feature of delusional disorder is that daily functioning is not markedly impaired, despite the presence of delusions. This distinguishes delusional disorder from schizophrenia and other psychotic disorders where functional deterioration is a core diagnostic requirement.

Key Diagnostic Features of Delusional Disorder

Preserved Functioning Despite Delusions

• Impact on functioning in delusional disorder is limited and consistent with the delusion or its ramifications, rather than showing marked global deterioration 1
• Appearance and behavior remain generally normal outside the delusional content 1
• This contrasts sharply with schizophrenia, which requires marked deterioration in social, occupational, and self-care functioning below pre-onset levels 2

Duration Requirement

• Delusional disorder requires at least 1 month's duration of delusions, not 3 months 1
• The DSM-III 6-month duration criterion identified more severe cases with poorer outcomes, but current diagnostic standards use the 1-month threshold 3
• ICD-10 uses a 3-month criterion, but DSM criteria (1-month) are more widely applied 3

Absence of Bizarre Behavior

• Behavior does NOT become bizarre in delusional disorder - this is actually a distinguishing feature that separates it from schizophrenia 1, 4
• The delusions themselves are non-bizarre, concerning experiences that could conceivably occur in real life (being followed, having a disease, being loved at a distance) 1

Single Symptom Presentation

• Delusional disorder is characterized by one or more delusions with relative absence of other psychopathology, not requiring at least two psychotic symptoms 1
• This contrasts with schizophrenia, which requires at least two psychotic symptoms from a list including delusions, hallucinations, disorganized speech, grossly disorganized/catatonic behavior, and negative symptoms 2

Exclusion of Substance-Related Causes

• Symptoms cannot be attributable to substance use disorder - this is an exclusion criterion, not a diagnostic feature 1
• The diagnosis specifically requires exclusion of substance-induced toxicity, medical disease, schizophrenia, and mood disorders 1

Clinical Implications

Recognition Challenges

• Patients typically lack awareness of the psychiatric nature of their condition and may present to non-psychiatric specialists (internists, dermatologists, lawyers, police) rather than psychiatrists 1
• The preserved functioning can make the disorder less obvious to clinicians compared to conditions with marked deterioration 1

Prognostic Considerations

• When adequately treated, delusional disorder has a reasonably good prognosis 5
• The DSM-III 6-month duration criterion was associated with more severe presentations including insidious onset, poor treatment response, chronic course, and poor psychosocial functioning 3