Difficulty: Intermediate | Prerequisites: Introductory Psychology, basic familiarity with DSM-5 diagnostic categories.
Differential diagnosis is the process of distinguishing one disorder from others that share overlapping symptoms. This set of notes covers the key differentials for anxiety disorders, trauma and stressor-related disorders, OCD-spectrum disorders, and dissociative disorders as listed in the DSM. Knowing which conditions mimic each other, and what separates them, is one of the most commonly tested skills in abnormal psychology.
Differential diagnosis
The systematic process of distinguishing a target disorder from other conditions that present with similar or overlapping symptoms. In simple terms, it is the clinician's checklist of "what else could this be?"
Comorbidity
The simultaneous presence of two or more diagnosable disorders in the same individual. Think of it as: a patient can have both depression and an anxiety disorder at the same time, and differential diagnosis helps determine whether the symptoms belong to one condition, the other, or both.
Rule-out
A candidate diagnosis that must be excluded before the primary diagnosis can be confirmed. In simple terms, a rule-out is a disorder that looks similar enough that you need evidence it is not present before you commit to your leading diagnosis.
Specifier
An extension appended to a diagnosis to indicate a particular subtype, severity level, or course (e.g. "with panic attacks," "mild," "in remission"). Think of it as a label that fine-tunes a diagnosis so two patients with the same disorder can be distinguished by the flavour of their presentation.
Primary vs. substance/medication-induced disorder
A primary disorder arises independently of substance use, while a substance/medication-induced disorder is directly caused by intoxication, withdrawal, or a medication's physiological effects. The distinction matters because treatment and prognosis differ: remove the substance, and the induced disorder often resolves.
Anxiety disorders share a common thread of excessive fear or worry, which is precisely why their differentials overlap so heavily. The table below lists each disorder alongside the conditions the DSM flags as look-alikes, followed by the key distinguishing feature.
Differentials: Social Anxiety Disorder, Substance/Medication-Induced Anxiety Disorder, PTSD, OCD, Depressive/Bipolar/Psychotic Disorders
GAD worry is diffuse and free-floating, spanning multiple domains. Social anxiety worry is confined to social evaluation. OCD worry attaches to specific intrusive thoughts with compulsions. PTSD worry follows a traumatic event and includes re-experiencing symptoms. If worry onset tracks substance use or withdrawal, rule out a substance-induced cause first.
Differentials: Normative Shyness, Agoraphobia, Panic Disorder, GAD, Separation Anxiety, Specific Phobias, Selective Mutism, MDD, Body Dysmorphic Disorder, Delusional Disorder, ASD, Avoidant Personality, ODD
The core feature is fear of negative evaluation in social or performance situations. Agoraphobia involves fear of situations where escape is difficult, not fear of judgment. Panic disorder involves unexpected panic attacks, whereas social anxiety attacks are cued by social exposure. Avoidant personality disorder overlaps significantly, but it is a pervasive, lifelong pattern rather than a situationally triggered anxiety.
Differentials: Agoraphobia, Panic Disorder, Social Anxiety, OCD, Trauma/Stressor-Related Disorders, Eating Disorders, Schizophrenia Spectrum Disorders
The fear is circumscribed to a particular object or situation (e.g. heights, animals, blood). Agoraphobia involves multiple situations and concern about being trapped. OCD involves avoidance driven by obsessional content, not a discrete phobic stimulus. If the avoidance is of food specifically, consider an eating disorder.
Differentials: Anxiety Disorder Due to Another Medical Condition, Substance/Medication-Induced Anxiety Disorder, Other Specified/Unspecified Anxiety Disorder, other mental disorders with panic attacks as an associated feature
Panic disorder requires recurrent, unexpected panic attacks plus persistent worry about future attacks or maladaptive behavioural change. If attacks are exclusively triggered by a known situation (social exposure, phobic object), the panic is better accounted for by the relevant anxiety disorder. Always rule out medical causes (hyperthyroidism, cardiac arrhythmias, pheochromocytoma) and substance effects.
Differentials: Specific Phobia (Situational Type), Separation Anxiety, Social Anxiety, PTSD, Acute Stress, Panic Disorder, MDD
Agoraphobia requires fear or avoidance of at least two of five situation types (public transport, open spaces, enclosed places, crowds, being outside the home alone). If the fear is limited to one situation, specific phobia is more appropriate. Separation anxiety is tied to attachment figures, not to the situations themselves.
Differentials: GAD, Panic Disorder, Agoraphobia, Conduct Disorder, Social Anxiety, PTSD, Illness Anxiety, Bereavement, Depressive/Bipolar Disorders, ODD, Psychotic Disorders, Personality Disorders
Distress centres on separation from attachment figures specifically. In agoraphobia the concern is about the situation; in separation anxiety it is about the person. GAD worry is broader and not attachment-focused. In children, refusal to attend school may look like conduct disorder or ODD, but the motivation differs: separation anxiety is fear-driven, not defiance-driven.
Differentials: Communication Disorders, Neurodevelopmental Disorders, Schizophrenia/Psychotic Disorders, Social Anxiety Disorder
The child can speak fluently in some contexts (typically at home) but consistently fails to speak in others (typically school). Communication disorders involve impaired ability rather than situational refusal. Social anxiety may co-occur, but selective mutism is diagnosed when the failure to speak is the prominent feature and goes beyond ordinary social reticence.
All disorders in this cluster require exposure to a traumatic or stressful event, which is the primary way they are distinguished from anxiety and depressive disorders that may present similarly.
Differentials: Acute Stress Disorder, Adjustment Disorder, Personality Disorders, Dissociative Disorders, Other PTSD-related conditions, Psychotic Disorders, MDD, Conversion Disorder, TBI
PTSD requires symptoms persisting beyond one month after trauma exposure. If symptoms last three days to one month, the diagnosis is acute stress disorder. Adjustment disorder is used when the stressor does not meet the trauma threshold (life-threatening, sexual violence, serious injury). Dissociative disorders may co-occur; the DSM includes a dissociative subtype of PTSD. Flashbacks can resemble psychotic hallucinations, but in PTSD the person typically recognises them as memories. TBI and PTSD frequently co-occur after physical trauma, and symptom overlap (concentration problems, irritability, sleep disturbance) makes separating them difficult.
Differentials: PTSD, Adjustment Disorder, Panic Disorder, Bipolar, Psychotic Disorders, Dissociative Disorders, OCD, TBI
Duration is the headline differentiator: acute stress disorder is diagnosed from 3 days to 1 month post-trauma. After 1 month, the diagnosis converts to PTSD if criteria are still met. Panic disorder involves unexpected attacks without a trauma link. OCD intrusions are ego-dystonic thoughts, not trauma re-experiencing.
Differentials: PTSD, Acute Stress Disorder, Normative Stress Reactions, MDD, Personality Disorders, Psychological Factors Affecting Other Medical Conditions
The stressor need not be traumatic (job loss, divorce, moving). Symptoms must be out of proportion to the stressor or cause significant functional impairment, but they do not meet the full criteria for another specific disorder. If full MDD criteria are met, MDD takes diagnostic precedence. A normative stress reaction is proportionate distress that does not impair functioning.
Differentials: Autism Spectrum Disorder, Depressive Disorders, Intellectual Disability
Presents with inhibited, emotionally withdrawn behaviour toward caregivers. Unlike ASD, the social deficits are attributable to a history of grossly inadequate caregiving and can improve with appropriate care. Unlike depression, the flat affect is specific to caregiver interactions, not pervasive mood.
Differentials: ADHD
The child approaches unfamiliar adults with reduced or absent reticence, which may look like ADHD impulsivity. The distinction is that disinhibited social engagement disorder requires a history of insufficient caregiving and the disinhibition is specifically social (approaching strangers), whereas ADHD impulsivity is domain-general.
This cluster shares the feature of repetitive behaviours or mental acts, but the content and function of those behaviours differ across diagnoses.
Differentials: Anxiety Disorders, MDD, Other OCD-Related Disorders, Eating Disorders, Tics/Stereotyped Movements, Psychotic Disorders, Other compulsive-like behaviours, OCPD
OCD involves intrusive, unwanted thoughts (obsessions) and repetitive behaviours or mental acts (compulsions) performed to neutralise anxiety. Anxiety disorder worry is about real-life concerns; OCD obsessions are typically irrational or exaggerated and recognised as such. Eating disorder rituals are confined to food and body image. Tics are involuntary movements, not purposeful anxiety-reducing behaviours. OCPD involves ego-syntonic perfectionism and rigidity (the person sees the traits as reasonable), unlike the ego-dystonic obsessions of OCD.
Differentials: Eating Disorders, Illness Anxiety Disorder, Other OCD-Related Disorders, MDD, Anxiety Disorders, Normal Appearance Concerns, Psychotic Disorders
BDD preoccupation focuses on perceived defects in physical appearance that are not observable or appear slight to others. In eating disorders, the concern is specifically about weight and body shape. In illness anxiety, the concern is about having a serious disease, not about appearance. Delusional disorder may apply if conviction about the defect reaches delusional intensity, but DSM allows BDD "with absent insight/delusional beliefs" as a specifier.
Differentials: Other Medical Conditions, Neurodevelopmental Disorders, Schizophrenia Spectrum, MDD, OCD, Neurocognitive Disorders
Persistent difficulty discarding possessions due to a perceived need to save them and distress associated with discarding. In OCD, hoarding-like behaviour is driven by specific obsessions (e.g. contamination fear prevents touching items to sort them). In neurocognitive disorders, accumulation results from cognitive decline rather than intentional saving. In depression, hoarding may reflect apathy and lack of energy rather than attachment to possessions.
Differentials: Other OCD-Related Disorders, Neurodevelopmental Disorders, Normative Hair Removal, Psychotic Disorders, Other Medical Conditions, Substance-Related Disorders
Recurrent pulling out of one's hair resulting in hair loss. Unlike OCD, there is typically no obsessional thought driving the behaviour; the pulling itself provides satisfaction or relief. Normative grooming (twirling, styling) does not result in hair loss. Medical causes (alopecia areata) must be excluded.
Differentials: Psychotic Disorder, Other OCD-Related Disorders, Neurodevelopmental Disorders, Somatic Symptom Disorders, Other Medical Conditions, Substance/Medication-Induced Disorders
Recurrent picking at one's skin resulting in skin lesions. In psychotic disorders, picking may be driven by delusions (e.g. believing insects are under the skin). Substance-induced picking (particularly stimulant use) must be ruled out. Dermatological conditions that cause actual skin lesions requiring attention are not excoriation disorder.
Dissociative disorders involve disruptions in consciousness, memory, identity, or perception. They frequently overlap with trauma-related, mood, and personality disorders, making differential diagnosis particularly challenging.
Differentials: Other Specified Dissociative Disorder, MDD, PTSD, Psychotic Disorders, Substance/Medication-Induced Disorders, Bipolar Disorders, Personality Disorders, Conversion Disorder, Seizure Disorder
DID involves the presence of two or more distinct personality states with recurrent gaps in recall. Hearing internal voices may resemble auditory hallucinations in psychosis, but in DID the voices are experienced as internal and are associated with identity alternation. Bipolar mood shifts are sustained episodes, not rapid identity switches. Borderline personality disorder involves identity disturbance, but not distinct personality states with amnesia barriers. Seizure disorders (particularly temporal lobe epilepsy) can produce dissociative-like states and should be medically ruled out.
Differentials: Factitious Disorder/Malingering, DID, Substance-Related Disorder, Neurocognitive Disorders, PTSD, Brain Injury, Seizure, Catatonia
Involves inability to recall important autobiographical information, usually of a traumatic or stressful nature. Unlike neurocognitive disorders, the memory loss is not due to a general medical condition and is typically reversible. In DID, amnesia co-occurs with identity alternation. Malingering should be considered when there is an external incentive (legal proceedings, financial gain). Substance-related blackouts involve amnesia during intoxication, which is a different mechanism.
Differentials: Other Dissociative Disorders, Illness Anxiety, OCD, Psychotic Disorders, MDD, Substance/Medication-Induced Disorders, Anxiety Disorders, Mental Disorders Due to Another Medical Condition
Persistent or recurrent experiences of detachment from one's mental processes or body (depersonalisation) or surroundings (derealisation), with intact reality testing. The preserved reality testing is the key separator from psychosis: the person knows their experience is not real. Brief episodes of depersonalisation during panic attacks do not warrant a separate diagnosis if they occur exclusively during panic. Medical conditions (temporal lobe epilepsy, migraine aura) can produce similar experiences and must be excluded.
Students often assume that panic attacks automatically mean Panic Disorder. They do not. Panic attacks can occur as a feature of nearly any anxiety, trauma, or mood disorder. Panic Disorder specifically requires recurrent unexpected attacks plus persistent concern about future attacks.
Students frequently conflate OCD with OCPD. OCD involves ego-dystonic intrusions the person wants to stop. OCPD involves ego-syntonic traits the person considers reasonable and even virtuous (orderliness, perfectionism, control). They are separate diagnoses in different DSM chapters.
Students often think Acute Stress Disorder is simply "mild PTSD." The two share symptoms, but the distinction is purely temporal: Acute Stress Disorder is diagnosed between 3 days and 1 month post-trauma, and PTSD after 1 month. The symptom threshold also differs.
Students sometimes treat Adjustment Disorder as a catch-all for any stress response. It is not. The stressor must be identifiable, the response must be clinically significant (out of proportion or functionally impairing), and the symptoms must not meet criteria for another specific disorder. If MDD criteria are met, MDD takes precedence.
⚠️ The PTSD vs. Acute Stress Disorder distinction (1-month threshold) is one of the most frequently tested differential pairs.
⚠️ GAD vs. Social Anxiety vs. OCD: exams commonly present a vignette and ask you to identify which disorder fits based on the content and scope of the worry. GAD = diffuse, multi-domain. Social Anxiety = social evaluation. OCD = specific intrusions with compulsions.
⚠️ Know the difference between OCD and OCPD cold. This appears on nearly every abnormal psychology exam.
⚠️ Depersonalisation/Derealisation Disorder vs. Psychosis: reality testing is preserved in the former. The person knows their experience feels unreal. In psychosis, that insight is absent.
⚠️ Adjustment Disorder is a residual category: it applies only when no other specific disorder's criteria are fully met. If an exam vignette meets MDD criteria after a stressor, the answer is MDD, not Adjustment Disorder.
⚠️ DID vs. Borderline Personality Disorder: both involve identity disturbance, but DID features distinct personality states with amnesia barriers, whereas BPD features an unstable self-image without dissociative identity alternation.
True or False: Panic attacks can only occur in Panic Disorder. (False. They can be a feature of many disorders.)
Fill in the blank: Acute Stress Disorder is diagnosed from ___ days to ___ month(s) after trauma. (3 days to 1 month.)
True or False: In Depersonalisation/Derealisation Disorder, the person has lost contact with reality. (False. Reality testing is preserved.)
Fill in the blank: Adjustment Disorder is diagnosed only when the symptoms do not meet full criteria for ___. (Another specific mental disorder, such as MDD.)
True or False: OCD and OCPD are different names for the same condition. (False. OCD involves ego-dystonic obsessions and compulsions; OCPD involves ego-syntonic rigidity and perfectionism.)
Q: A patient presents with excessive worry about multiple life domains (finances, health, relationships) lasting over six months. They deny specific intrusive thoughts or compulsions. What is the most likely diagnosis, and what would you rule out?
A: Generalised Anxiety Disorder. Rule out Social Anxiety Disorder (worry would be confined to social evaluation), OCD (would involve specific obsessions and compulsions), MDD (would require depressed mood or anhedonia as core features), and substance/medication-induced anxiety.
Q: A soldier returns from deployment and reports flashbacks, hypervigilance, and emotional numbing three weeks after a combat incident. What diagnosis applies now, and what will it become if symptoms persist past one month?
A: Acute Stress Disorder (symptoms present between 3 days and 1 month post-trauma). If symptoms persist beyond one month, the diagnosis changes to PTSD.
Q: A patient reports that the world around them feels dreamlike and unreal, as though they are watching their life from behind glass. They acknowledge this feeling is strange and know it is not actually the case. What is the most likely diagnosis?
A: Depersonalisation/Derealisation Disorder. The key feature is preserved reality testing, ruling out psychotic disorders.
Q: A child consistently refuses to speak at school despite speaking fluently at home. There is no evidence of a communication disorder or ASD. What is the most likely diagnosis?
A: Selective Mutism. The ability to speak is intact, and the failure to speak is situationally specific.
Q: A patient reports persistent difficulty discarding newspapers, clothing, and packaging materials. They feel distressed when asked to throw things away and say each item might be useful someday. They deny any obsessional thoughts about contamination or harm. What is the most likely diagnosis?
A: Hoarding Disorder. The absence of obsessional content (contamination, harm) distinguishes this from OCD-driven hoarding, where accumulation is secondary to avoidance of feared outcomes.
Q: How do you differentiate DID from Borderline Personality Disorder when both involve identity disturbance?
A: DID involves the presence of two or more distinct personality states with recurrent gaps in autobiographical recall (amnesia barriers). BPD involves chronic instability of self-image and interpersonal relationships without distinct identity states or dissociative amnesia.
This material connects directly to the mood disorders and psychotic disorders covered in Part 2 of these notes, because many anxiety and trauma differentials include MDD, bipolar disorder, and schizophrenia spectrum conditions. Understanding personality disorders (Part 2) is also essential, since avoidant, borderline, and obsessive-compulsive personality disorders frequently appear as differentials for anxiety and OCD-spectrum conditions.
The substance-related disorders covered in Part 3 are relevant because substance/medication-induced anxiety, OCD, and dissociative presentations must be ruled out before a primary diagnosis is made.
Related Terms / Search Tags
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