Two people can sit in the same waiting room with the same diagnosis and share almost no symptoms. And two people that share many of the same systems might have a different diagnosis.
Under the DSM-IV, there were 79,794 distinct symptom combinations that all qualified as post-traumatic stress disorder; the DSM-5 revision expanded that to 636,120 (Galatzer-Levy & Bryant, 2013). A single diagnostic label, in other words, can describe more than half a million clinically different people. This is the central problem of differential diagnosis: the categories we use are broad, the symptoms inside them overlap heavily with neighbouring conditions, and the same presentation can have several plausible explanations.
Differential diagnosis is the disciplined process of working through those possibilities and identifying which condition (or combination of conditions) best accounts for a person’s presentation, and which can be reasonably ruled out. It is among the most consequential reasoning a clinician does, because the diagnosis shapes everything that follows: the formulation, the treatment plan, medication decisions, funding and supports, and how the person comes to understand themselves.
This article is the overview. It explains what differential diagnosis is, why mental health conditions are so easily confused, and how to think about the disorders most often tangled together—autism and ADHD, the anxiety subtypes, OCD and body dysmorphic disorder, borderline personality disorder and complex trauma, bipolar disorder, and more. Each section links to a deeper article and to the validated assessments on NovoPsych that support structured diagnostic decision-making.
In mental health, differential diagnosis is the systematic comparison of competing diagnostic explanations for a person’s presenting problems. It involves generating a list of candidate conditions consistent with the symptoms, then using history, clinical interview, collateral information, and psychometric assessment to weigh them against one another—confirming some, ruling out others, and recognising where two or more conditions genuinely co-occur.
It is not the same as screening. A screening tool such as the GAD-7 or the Kessler Psychological Distress Scale (K10) tells you that something warrants attention; differential diagnosis tells you what. A high GAD-7 score, for example, is sensitive but not specific—elevated scores also appear in panic disorder, social anxiety, and PTSD. The screen opens the question; the differential answers it.
Three structural features of psychiatric classification make this difficult, and they are worth naming explicitly because they recur throughout this article (Kotov et al., 2017):
Good differential diagnosis is the clinician’s discipline for managing these three problems rather than being managed by them.
It is worth dwelling on heterogeneity because it explains so much downstream confusion. Imagine the diagnostic criteria for a disorder as a checklist where a person needs, say, five of nine symptoms. The number of qualifying combinations grows combinatorially. Two clients can each meet criteria for borderline personality disorder while sharing only one or two symptoms—one presenting with chronic emptiness, identity disturbance, and dissociation, the other with impulsivity, anger, and self-harm. They have the same diagnosis and almost nothing in common in the room.
The clinical implication is that the diagnostic label is a starting point, not an endpoint. Two people with “the same” condition may need substantially different formulations and treatments—which is why NovoPsych pairs categorical assessment with dimensional and formulation-based measures.
The DSM-5-TR and ICD-11 are categorical systems: a person either does or does not meet criteria for a disorder, with a threshold drawn somewhere along what is usually an underlying continuum. Categorical systems have real virtues—they are communicable, they map onto treatment guidelines and funding, and they support clinical decisions that are themselves categorical (to medicate or not, to refer or not).
But the thresholds are, to a degree, arbitrary. Someone with four symptoms of a five-symptom disorder is categorised identically to someone with none, despite being far more similar to the person who has five. This is the “boundary problem,” and it is one reason comorbidity is so common: when a single underlying dimension (say, negative affectivity) drives several categorical disorders, a person elevated on that dimension will cross several thresholds at once and accumulate multiple diagnoses that are really facets of one problem (Hyman, 2010; Kotov et al., 2017).
A further complication is time. Differential diagnosis requires distinguishing transient states from enduring traits. A person who is acutely depressed may appear inattentive, socially withdrawn, indecisive, and emotionally labile—a presentation that can mimic ADHD, social anxiety, or a personality disorder. Until the mood episode resolves, the underlying picture is obscured.
This is why developmental history is decisive. ADHD and autism are neurodevelopmental conditions with onset in childhood; a presentation of inattention that began at age 40 is far more likely to be depression, anxiety, or trauma than emerging ADHD. Conversely, a personality-disorder diagnosis should not be made on the basis of behaviour confined to an acute episode. Tools that track symptoms over time—such as paired outcome-monitoring measures—help separate state from trait by showing what persists once an acute episode lifts.
The remainder of this guide works through the pairings clinicians most often need to disentangle. Each will become a dedicated article; this section gives the essential logic and the assessments that help.
The autism–ADHD relationship is the clearest illustration of how diagnostic rules shape clinical reality. Until 2013, the DSM explicitly prohibited diagnosing both: autism was an exclusion criterion for ADHD. The DSM-5 removed that exclusion, formally recognising for the first time that the two conditions co-occur (American Psychiatric Association, 2013; Antshel & Russo, 2019). Estimates of co-occurrence are high and vary considerably by sample and method—commonly cited figures range from around 30% to over 50% of autistic people also meeting criteria for ADHD (Antshel & Russo, 2019; Hours, Recasens & Baleyte, 2022).
The popular term “AuDHD” reflects a growing recognition that this combined presentation may be its own coherent profile rather than two conditions awkwardly stapled together. The overlap is genuine at the symptom level: both involve executive dysfunction, both can present with social difficulty, and inattention can look like the cognitive disengagement seen in autism. The discriminating features lie in the why—social difficulty in ADHD is often driven by impulsivity and inattention, whereas in autism it reflects differences in social communication and a preference for predictability.
Differentiating them—and detecting co-occurrence—benefits from structured measurement using complementary tools: the Adult ADHD Self-Report Scale (ASRS) alongside autism measures such as the Autism Spectrum Quotient (AQ), the Ritvo Autism Asperger Diagnostic Scale–Revised (RAADS-R), or the Comprehensive Autistic Trait Inventory (CATI). Because many autistic adults—particularly women—mask their traits, the Camouflaging Autistic Traits Questionnaire (CAT-Q) is valuable for detecting presentations that screening tools miss, and the Executive Skills Questionnaire–Revised (ESQ-R) helps map the executive profile common to both. (See the dedicated guide: Autism vs ADHD — differential diagnosis.)
In-depth guide: Autism vs ADHD: A Clinician’s Guide to Differential Diagnosis
Anxiety is not one thing, and the differential within anxiety is as important as the differential between anxiety and other conditions. Generalised anxiety, panic, and social anxiety share autonomic arousal but differ in their cognitive core: generalised anxiety centres on uncontrollable worry across domains, panic on catastrophic interpretation of bodily sensations, and social anxiety on fear of negative evaluation.
This matters for measurement. The GAD-7 is excellent for worry-based generalised anxiety but, by design, will also flag panic and social anxiety without distinguishing them. The DASS-21 anxiety subscale, by contrast, captures physiological/panic-spectrum arousal more than worry. Where social anxiety is suspected, interaction- and performance-specific measures (such as the Social Interaction Anxiety Scale) sharpen the picture. NovoPsych’s anxiety assessment library groups these tools so the right instrument matches the suspected subtype.
A particularly important differential here is social anxiety versus autism. Both involve social avoidance and discomfort, but the mechanism differs: in social anxiety the person typically wants social connection but fears judgement, whereas in autism the social difficulty is rooted in communication differences and may not be accompanied by the same fear of evaluation. Misreading one for the other sends treatment in the wrong direction. (See the dedicated guide: Social anxiety vs autism.)
In-depth guide: Social Anxiety vs Autism: A Clinician’s Guide to Differential Diagnosis
Obsessive-compulsive disorder and body dysmorphic disorder sit within the same DSM-5 chapter—the obsessive-compulsive and related disorders—and they share a common architecture of intrusive thoughts and repetitive behaviours (Phillips et al., 2010). In BDD, the obsessional focus is a perceived defect in appearance, and the compulsions (mirror-checking, reassurance-seeking, camouflaging) are organised around it. The conditions co-occur frequently and share genetic and clinical features, which is precisely why they are easy to confuse.
The discriminating question is the content and insight of the preoccupation. Where the intrusive concern is appearance-specific and ego-syntonic, BDD is more likely; where obsessions range across contamination, harm, symmetry, or taboo themes, OCD is the better fit. The Obsessive-Compulsive Inventory–Revised (OCI-R) maps OCD symptom domains and supports tracking, and forms a useful anchor when working through this differential. (See the dedicated guide: OCD vs body dysmorphic disorder.)
In-depth guide: OCD vs BDD: A Clinician’s Guide to Differential Diagnosis
Few differentials are as clinically charged—or as consequential—as borderline personality disorder versus complex PTSD. The ICD-11’s formal introduction of complex PTSD (CPTSD) crystallised a long-standing question: how much of what we call BPD is, in fact, the sequela of chronic interpersonal trauma? CPTSD comprises the three core PTSD clusters plus “disturbances in self-organisation”—affect dysregulation, negative self-concept, and disturbed relationships—which overlap substantially with BPD’s affective instability, identity disturbance, and unstable relationships (Cloitre et al., 2013; Brewin et al., 2017).
The distinctions that matter: CPTSD requires a trauma history and is anchored in re-experiencing and avoidance, whereas BPD does not require trauma and is characterised by frantic efforts to avoid abandonment, identity disturbance, and recurrent self-harm or suicidality that are not core to CPTSD. The two can also co-occur. Structured measurement helps: the International Trauma Questionnaire (ITQ) was purpose-built to distinguish PTSD from CPTSD, while the McLean Screening Instrument for BPD (MSI-BPD) and the Borderline Symptom List (BSL-23) quantify borderline symptomatology. A trauma history can be documented with the Life Events Checklist (LEC-5) and the Adverse Childhood Experiences Questionnaire (ACE-Q). (See the dedicated guide: BPD vs complex PTSD.)
In-depth guide: BPD vs CPTSD: Distinguishing Borderline Personality Disorder from Complex PTSD
In-depth guide: Were Many “Borderline” Women Actually Autistic? Autism, BPD and Complex PTSD
The bipolar–borderline differential carries some of the highest stakes in psychiatry, because the treatments diverge sharply: bipolar disorder responds to mood stabilisers, while BPD is primarily treated with structured psychotherapy. Misdiagnosing one as the other can mean years of ineffective or inappropriate treatment.
Both feature mood instability, but the temporal grain differs. Bipolar mood episodes last days to weeks and are relatively autonomous of circumstance; borderline affective instability shifts within hours and is typically reactive to interpersonal events. Self-report screeners can help differentiate the two—the Mood Disorder Questionnaire (MDQ) for bipolar and the MSI-BPD for borderline features—though screening must always be confirmed by clinical interview (Zimmerman & Morgan, 2013). NovoPsych’s overview of bipolar assessment questionnaires covers screening and outcome monitoring across the mood-disorder spectrum. (See the dedicated guide: Bipolar disorder vs BPD.)
In-depth guide: Bipolar Disorder vs Borderline Personality Disorder
ADHD and trauma are frequently confused, particularly in children and in adults with childhood adversity. The overlap is real: difficulty concentrating, restlessness, irritability, emotional dysregulation, and sleep disturbance appear in both ADHD and PTSD/CPTSD. A traumatised child who is hypervigilant and unable to settle in class can easily be read as inattentive and hyperactive.
The differentiating logic again turns on onset and mechanism. ADHD is neurodevelopmental and pervasive across settings from early childhood; trauma-related inattention typically follows an identifiable adversity and is accompanied by re-experiencing, avoidance, and hyperarousal. The two can co-occur, and untreated ADHD is itself a risk factor for trauma exposure. Differentiation is supported by pairing the ASRS with trauma measures such as the PTSD Checklist for DSM-5 (PCL-5), the ITQ, and the Impact of Event Scale–Revised (IES-R). NovoPsych’s guide to trauma psychometric scales details the full toolkit. (See the dedicated guide: ADHD vs PTSD/CPTSD.)
In-depth guide: ADHD vs PTSD and Complex PTSD: A Clinician’s Guide to Differential Diagnosis
ADHD and anxiety are among the most commonly confused adult presentations—and among the most commonly co-occurring. Restlessness, difficulty concentrating, and a racing mind feature in both. But the direction of causation differs: in anxiety, concentration fails because attention is captured by worry; in ADHD, the attentional difficulty is primary and pervasive, present even when the person is calm and engaged with something they enjoy. Anxiety can also be secondary to undiagnosed ADHD, as years of missed deadlines and underperformance generate genuine, well-founded worry.
Distinguishing them requires examining whether attentional difficulties predate and exist independently of anxiety, using the ASRS and GAD-7 together and attending closely to developmental history. (See the dedicated guide: ADHD vs anxiety.)
In-depth guide: ADHD vs Anxiety: A Clinician’s Guide to Differential Diagnosis
The table below summarises the overlapping features and key discriminators across the differentials covered above. It is a heuristic aid, not a diagnostic instrument—every differential ultimately rests on a thorough clinical interview.
| Differential | What they share | Key discriminator | Helpful NovoPsych measures |
|---|---|---|---|
| Autism vs ADHD | Executive dysfunction, social difficulty, inattention | Social difficulty: communication difference (autism) vs impulsivity/inattention (ADHD); both can co-occur ("AuDHD") | ASRS, AQ, RAADS-R, CAT-Q |
| Social anxiety vs autism | Social avoidance and discomfort | Desire for connection + fear of judgement (social anxiety) vs communication difference (autism) | Anxiety scales, AQ |
| Generalised vs other anxiety | Autonomic arousal | Worry (GAD) vs bodily catastrophising (panic) vs evaluation fear (social) | GAD-7, DASS-21 |
| OCD vs BDD | Intrusive thoughts, compulsions | Appearance-specific preoccupation (BDD) vs broad obsessional themes (OCD) | OCI-R |
| BPD vs CPTSD | Affect dysregulation, relationship difficulty, poor self-concept | Trauma history + re-experiencing (CPTSD) vs abandonment fear + identity disturbance (BPD) | ITQ, MSI-BPD, BSL-23 |
| Bipolar vs BPD | Mood instability | Episodes of days–weeks, autonomous (bipolar) vs hourly, reactive (BPD) | MDQ, MSI-BPD |
| ADHD vs PTSD/CPTSD | Inattention, restlessness, dysregulation | Lifelong + pervasive (ADHD) vs post-trauma + re-experiencing (PTSD) | ASRS, PCL-5, ITQ |
| ADHD vs anxiety | Restlessness, poor concentration | Attention fails even when calm (ADHD) vs attention captured by worry (anxiety) | ASRS, GAD-7 |
If categorical diagnosis is so beset by comorbidity, heterogeneity, and unreliability, why do so many of these conditions cluster together? The Hierarchical Taxonomy of Psychopathology (HiTOP) offers a compelling answer. Rather than treating disorders as discrete categories, HiTOP organises psychopathology as a hierarchy of empirically-derived dimensions, built from the observed covariation among symptoms (Kotov et al., 2017; 2021). At the top sits a general factor of psychopathology (the “p factor”); below it, broad spectra such as internalising, externalising, and thought disorder; and below those, narrower components and individual symptoms.
The implications for differential diagnosis are direct. Much of the comorbidity that frustrates categorical diagnosis is, on the dimensional view, an artefact of drawing category boundaries through what are really shared underlying dimensions. The reason anxiety, depression, and PTSD co-occur so often is that they load on a common internalising spectrum; the reason ADHD, conduct problems, and substance use cluster is a shared externalising dimension. Seen this way, the clinician’s task is less “which single box does this person belong in?” and more “where does this person sit on the relevant dimensions, and which are clinically actionable?”
This is not merely theoretical. Dimensional measurement reduces the heterogeneity problem (by scoring the amount of a trait rather than forcing a yes/no), improves reliability (continuous scores are more stable than threshold crossings), and aligns with how treatments actually work. NovoPsych’s Brief Hierarchical Taxonomy of Psychopathology (B-HiTOP) operationalises this framework for clinical use, giving a transdiagnostic profile that complements—rather than replaces—categorical diagnosis. The dimensional and categorical approaches are best understood as two lenses on the same person: the dimension tells you the shape and severity of the problem; the category translates it into the language of treatment guidelines, funding, and communication.
It would be a mistake to read the dimensional critique as an argument against diagnosis. Diagnosis is here to stay, and for good reason—getting it right changes lives, and getting it wrong causes harm.
The clearest case is where diagnosis dictates treatment. Bipolar disorder misdiagnosed as unipolar depression and treated with antidepressant monotherapy can precipitate mania; correctly identified, it responds to mood stabilisation. ADHD that is recognised can be treated with stimulant medication that is, by the standards of psychiatry, remarkably effective; left undiagnosed, it compounds across a lifetime into academic underachievement, occupational instability, and secondary anxiety and depression (Faraone et al., 2021). Personality disorders, when accurately identified, open the door to evidence-based psychotherapies; when missed, the person cycles through services collecting partial diagnoses and partial treatments.
There is also the problem of diagnostic overshadowing—the tendency to attribute every new symptom to an existing diagnosis (Reiss, Levitan & Szyszko, 1982). Once a person is labelled with an intellectual disability, autism, or a severe mental illness, clinicians may reflexively explain away new presentations as part of the known condition, missing treatable comorbidities. Autistic people, who are at substantially elevated risk for anxiety, depression, and OCD, are especially vulnerable to having those treatable conditions overlooked. Careful differential diagnosis—and a willingness to keep asking “what else could this be?”—is the antidote.
The goal, then, is not to abandon diagnostic categories but to hold them well: to use them as tested hypotheses rather than fixed conclusions, to support them with structured assessment rather than impression alone, and to remain alert to comorbidity, heterogeneity, and the limits of any single label. That is what differential diagnosis, done properly, achieves.
Differential diagnosis is reasoning, not a questionnaire—but structured measurement makes the reasoning sharper, more reliable, and more transparent to the people we assess. NovoPsych’s library of over 150 validated instruments spans the diagnostic, formulation, and outcome-monitoring measures referenced throughout this guide, with automatic scoring and norm-referenced reporting that supports defensible diagnostic decisions.
The articles linked above go deeper into each specific differential. Together they form a connected resource for one of the hardest and most important tasks in clinical practice: working out not just that someone is struggling, but precisely how—and what will help.
American Psychiatric Association. (2013). Diagnostic and Statistical Manual of Mental Disorders (5th ed.). APA.
American Psychiatric Association. (2022). Diagnostic and Statistical Manual of Mental Disorders (5th ed., text rev.). APA.
Antshel, K. M., & Russo, N. (2019). Autism spectrum disorders and ADHD: Overlapping phenomenology, diagnostic issues, and treatment considerations. Current Psychiatry Reports, 21(5), 34.
Baron-Cohen, S., Wheelwright, S., Skinner, R., Martin, J., & Clubley, E. (2001). The Autism-Spectrum Quotient (AQ): Evidence from Asperger syndrome/high-functioning autism, males and females, scientists and mathematicians. Journal of Autism and Developmental Disorders, 31(1), 5–17.
Blevins, C. A., Weathers, F. W., Davis, M. T., Witte, T. K., & Domino, J. L. (2015). The Posttraumatic Stress Disorder Checklist for DSM-5 (PCL-5): Development and initial psychometric evaluation. Journal of Traumatic Stress, 28(6), 489–498.
Bohus, M., Kleindienst, N., Limberger, M. F., et al. (2009). The short version of the Borderline Symptom List (BSL-23): Development and initial data on psychometric properties. Psychopathology, 42(1), 32–39.
Brewin, C. R., Cloitre, M., Hyland, P., et al. (2017). A review of current evidence regarding the ICD-11 proposals for diagnosing PTSD and complex PTSD. Clinical Psychology Review, 58, 1–15.
Cloitre, M., Garvert, D. W., Brewin, C. R., Bryant, R. A., & Maercker, A. (2013). Evidence for proposed ICD-11 PTSD and complex PTSD: A latent profile analysis. European Journal of Psychotraumatology, 4, 20706.
Faraone, S. V., Banaschewski, T., Coghill, D., et al. (2021). The World Federation of ADHD International Consensus Statement: 208 evidence-based conclusions about the disorder. Neuroscience & Biobehavioral Reviews, 128, 789–818.
Foa, E. B., Huppert, J. D., Leiberg, S., et al. (2002). The Obsessive-Compulsive Inventory: Development and validation of a short version. Psychological Assessment, 14(4), 485–496.
Galatzer-Levy, I. R., & Bryant, R. A. (2013). 636,120 ways to have posttraumatic stress disorder. Perspectives on Psychological Science, 8(6), 651–662.
Hirschfeld, R. M. A., Williams, J. B. W., Spitzer, R. L., et al. (2000). Development and validation of a screening instrument for bipolar spectrum disorder: The Mood Disorder Questionnaire. American Journal of Psychiatry, 157(11), 1873–1875.
Hours, C., Recasens, C., & Baleyte, J. M. (2022). ASD and ADHD comorbidity: What are we talking about? Frontiers in Psychiatry, 13, 837424.
Hyman, S. E. (2010). The diagnosis of mental disorders: The problem of reification. Annual Review of Clinical Psychology, 6, 155–179.
Kotov, R., Krueger, R. F., Watson, D., et al. (2017). The Hierarchical Taxonomy of Psychopathology (HiTOP): A dimensional alternative to traditional nosologies. Journal of Abnormal Psychology, 126(4), 454–477.
Kotov, R., Krueger, R. F., Watson, D., et al. (2021). The Hierarchical Taxonomy of Psychopathology (HiTOP): A quantitative nosology based on consensus of evidence. Annual Review of Clinical Psychology, 17, 83–108.
Phillips, K. A., Stein, D. J., Rauch, S. L., et al. (2010). Should an obsessive-compulsive spectrum grouping of disorders be included in DSM-V? Depression and Anxiety, 27(6), 528–555.
Regier, D. A., Narrow, W. E., Clarke, D. E., et al. (2013). DSM-5 field trials in the United States and Canada, Part II: Test-retest reliability of selected categorical diagnoses. American Journal of Psychiatry, 170(1), 59–70.
Reiss, S., Levitan, G. W., & Szyszko, J. (1982). Emotional disturbance and mental retardation: Diagnostic overshadowing. American Journal of Mental Deficiency, 86(6), 567–574.
Ritvo, R. A., Ritvo, E. R., Guthrie, D., et al. (2011). The Ritvo Autism Asperger Diagnostic Scale–Revised (RAADS-R): A scale to assist the diagnosis of autism spectrum disorder in adults. Journal of Autism and Developmental Disorders, 41(8), 1076–1089.
Zanarini, M. C., Vujanovic, A. A., Parachini, E. A., et al. (2003). A screening measure for BPD: The McLean Screening Instrument for Borderline Personality Disorder (MSI-BPD). Journal of Personality Disorders, 17(6), 568–573.
Zimmerman, M., & Morgan, T. A. (2013). The relationship between borderline personality disorder and bipolar disorder. Dialogues in Clinical Neuroscience, 15(2), 155–169.