Differential Diagnosis

Differential Diagnosis: Ruling Things Out Before You Rule One In

Knowing criteria is only half the job. The other half is the client whose symptoms fit two or three disorders. That is differential diagnosis: deciding which of several look-alike conditions explains the picture. The EPPP lists it as its own knowledge area, along with bringing non-psychological information (medical, drug, and record data) into an assessment. This lesson teaches the order to think in, the medical and drug look-alikes, the classic look-alike pairs, and how culture changes the answer. Full criteria live in the diagnosis lessons.

Why This Matters for Psychologists

DSM-5-TR says checking off symptoms is not enough. Diagnosis needs a careful history and clinical judgment (American Psychiatric Association [APA], 2022). A wrong call has a cost. Treat "panic disorder" when the cause is an overactive thyroid, and the real illness goes untreated.

A car's check-engine light tells you something is wrong, not what. A good mechanic checks the dangerous and easy-to-fix causes before taking the engine apart.

The Six-Step Approach

Michael First's DSM-5-TR Handbook of Differential Diagnosis, a companion book to DSM-5-TR, builds the process around six steps taken in this order (First, 2024; LeBano, 2014). DSM-5-TR itself doesn't number these steps. It supplies the parts: a Differential Diagnosis section for each disorder and the rules for substance-induced, medical, and other specified diagnoses (APA, 2022).

Step 1 comes first because diagnosis rests on an honest report. If the symptoms are false, everything built on them is wrong. First also warned against treating clients like hostile witnesses (LeBano, 2014). Suspect malingering when you see a medicolegal context, a big gap between claimed disability and what you observe, poor cooperation, or antisocial personality disorder. Malingering has an external incentive and is not a mental disorder (APA, 2022). Factitious disorder involves deception without an obvious external reward. Detection methods are covered in the Trauma/Stressor-Related, Dissociative, and Somatic Symptom Disorders lesson.

Step 2 asks about drugs, medications, toxins, and withdrawal. DSM-5 renamed these disorders "substance/medication-induced" to stress that prescribed medications, not just drugs of abuse, can cause psychiatric symptoms (APA, 2022).

Step 3 asks whether a physical illness is acting on the brain. DSM-5-TR says "another medical condition," because mental disorders are medical conditions too (APA, 2022).

Step 4 names the specific disorder. DSM-5 replaced "primary" with "independent" for a disorder not caused by a substance or medical condition (APA, 2022). On the exam, treat the two words as the same idea.

Step 5 handles presentations that fall short of a full disorder. If symptoms are a maladaptive response to a stressor, use adjustment disorder (LeBano, 2014). If not, use other specified (you record why criteria aren't met) or unspecified (you don't, as in an emergency room) (APA, 2022).

Step 6 draws the line with ordinary life. Most disorders require clinically significant distress or impairment. Uncomplicated bereavement or a phase-of-life problem can be a focus of care without being a mental disorder (APA, 2022).

Decision-Tree Thinking

First's handbook pairs the steps with 30 symptom-oriented decision trees and 67 differential diagnosis tables (First, 2024). A tree starts from the presenting symptom and asks yes-or-no questions until one answer is left. Here is a simple tree for depressed mood, built from DSM-5-TR's differential sections (APA, 2022):

1. Feigned? Malingering or factitious disorder.
2. Caused by a substance or medication? Substance/medication-induced depressive disorder.
3. Caused by a medical condition, such as hypothyroidism? Depressive disorder due to another medical condition.
4. Ever a manic episode? Bipolar I. Ever a hypomanic episode but never mania? Bipolar II.
5. Psychosis only during mood episodes? A mood disorder with psychotic features. Psychosis for 2+ weeks with no mood episode? Think schizoaffective disorder or schizophrenia (see the pairs table below).
6. Depressed mood more days than not for 2 years? Persistent depressive disorder (PDD). Add MDD if its criteria are also met.
7. Full major depressive episode? MDD. Short of criteria after a stressor? Adjustment disorder. Short of criteria with no stressor? Other specified or unspecified depressive disorder.
8. No significant distress or impairment? No disorder.

It works like the gates at a recycling plant. Each gate pulls out one kind of item, so whatever reaches the end is what nothing else could explain.

Steps 2 and 3: Medical and Substance Look-Alikes

Three Tests for a Medical Cause

DSM-5-TR offers three considerations (APA, 2022):

• Biological plausibility: the condition is known to cause these symptoms, like hyperthyroidism causing anxiety.
• Temporality: symptoms start, worsen, and ease along with the medical condition. This gives the most certainty.
• Typicality: features are unusual for the independent disorder, such as an odd age at onset or visual or olfactory hallucinations.

For substances, evidence of an independent disorder includes symptoms that began before the substance use, symptoms that last about 1 month after acute withdrawal or heavy intoxication ends, or past episodes without substances. Psychosis during or within 4 weeks of intoxication or withdrawal points to a substance-induced psychotic disorder (APA, 2022).

Medical Mimics

DSM-5-TR names each of these as a physical cause, or a close look-alike, of psychiatric symptoms (APA, 2022).

Thyroid and other endocrine details are in the Neurological and Endocrine Disorders lesson.

Medication and Substance Mimics

Ask about every drug, prescribed or not. These come from DSM-5-TR unless another source is named (APA, 2022).

Caveat on beta-blockers: a meta-analysis of 15 trials found no significant rise in depressive symptoms, only small increases in fatigue and sexual dysfunction (Ko et al., 2002).

When to Refer for Labs, Imaging, or a Medical Exam

You don't diagnose the medical condition yourself. A "due to another medical condition" diagnosis needs that condition established by history, physical exam, or lab findings (APA, 2022). Your job is to spot red flags and refer:

• Late or odd onset: first panic attacks after age 45, or attacks with vertigo, passing out, loss of bladder control, slurred speech, or amnesia.
• Unusual hallucinations: visual or olfactory. Olfactory hallucinations suggest temporal lobe epilepsy.
• New psychosis in an older adult: an estimated 60% have a medical cause.
• Fluctuating attention and awareness: possible delirium, which always has a physical cause and takes priority.
• A clear time link to a new illness, medication, or dose change.

DSM-5-TR names tests that help: serum calcium (parathyroid disease can cause panic), a Holter heart monitor (arrhythmias), urine or blood drug screens, and sleep studies (APA, 2022). In a classic study of 658 outpatients, 9.1% had a medical illness causing their psychiatric symptoms, and 46% of those illnesses were unknown to patient and doctor. The authors treated visual hallucinations as medical until proven otherwise (Hall et al., 1978). Caveats: no single hallucination proves a medical cause, since visual hallucinations also occur in schizophrenia and bipolar disorder (APA, 2022). And there is still no agreed standard workup, even for first-episode psychosis (Freudenreich et al., 2009). How hearing and vision problems can lower test scores is covered in the Clinical Tests lesson.

Integrating Records and Collateral

The client is one source, not the only one. DSM-5-TR recommends information from family and other third parties when judging impairment. It also allows a provisional diagnosis while you wait for an informant or records (APA, 2022). Collateral often decides the case:

• Bipolar II: clients may not see hypomania as a problem, so relatives often establish it.
• ADHD vs. substance use: records or informants showing ADHD before the substance use may be essential.
• Borderline personality disorder: document an early-onset, long-standing pattern. Don't diagnose from one snapshot taken during a mood episode.
• Young children and trauma: parents may minimize symptoms, and teachers may not know about the trauma.
• Intellectual disability: knowledgeable informants are essential for spotting irritability, sleep, or eating problems.
• Factitious disorder: medical records and interviews with others can reveal deception.

Collateral is like video replay in sports. One camera can miss the foul. A second angle settles it.

High-Yield Look-Alike Pairs

Each pair below turns on one feature, drawn from DSM-5-TR's differential diagnosis sections (APA, 2022). Full criteria live in the diagnosis lessons.

Two more traps, taught in full in the Trauma/Stressor-Related, Dissociative, and Somatic Symptom Disorders lesson: FND is not a diagnosis of exclusion (it needs positive signs, such as Hoover's sign), and somatic symptom disorder doesn't require that symptoms lack a medical cause (APA, 2022). Pseudodementia is compared in depth in the Neurocognitive Disorders lesson.

Worked Examples

Case 1. An attorney refers a 40-year-old man after a car crash. He says he can't learn anything new. His wife says that since the crash he has learned a new phone app and still pays the family bills.

• Reasoning: Step 1. A medicolegal referral and a gap between claimed disability and observed function are red flags.
• Answer: Strongly suspect malingering, and check effort with validity testing before diagnosing a neurocognitive disorder. The same faking with no external reward would suggest factitious disorder.

Case 2. A 26-year-old woman has panic attacks that began two days after she stopped heavy daily drinking. The attacks, not the shakes, are what brought her in. They faded within two weeks.

• Reasoning: Step 2. Withdrawal from depressants like alcohol can trigger panic attacks. Panic disorder needs attacks that continue long after withdrawal ends.
• Answer: Alcohol-induced anxiety disorder, with onset during withdrawal, not panic disorder. (If the panic were minor, plain alcohol withdrawal would cover it.)

Case 3. A 58-year-old woman began high-dose prednisone for a severe asthma flare 10 days ago. Now she sleeps 3 hours, talks fast, and spends wildly. She has no mood history.

• Reasoning: Step 2. The episode began days after a high-dose steroid start, with no prior episodes. Asthma itself doesn't cause mania.
• Answer: Substance/medication-induced bipolar and related disorder (corticosteroid-induced).

Case 4. A 30-year-old started an SSRI for major depression. Two weeks later she has elevated mood, little need for sleep, fast speech, and grandiosity. She keeps working, with no marked impairment. The full syndrome continues a month after the SSRI is stopped.

• Reasoning: Hypomania that persists at a full level past the drug's effects counts as true bipolar disorder. Edginess or irritability alone would not.
• Answer: Bipolar II disorder (hypomania plus a past major depressive episode).

Case 5. A 52-year-old man has his first-ever panic attacks. During one, the room spun and his speech slurred.

• Reasoning: Step 3. Onset after 45 plus atypical symptoms point to a possible medical or substance cause.
• Answer: The best next step is referral for a medical evaluation, before diagnosing panic disorder.

Case 6. A 68-year-old woman with no personal or family psychiatric history develops persecutory delusions and keeps smelling burning rubber.

• Reasoning: Typicality red flags: late onset and an olfactory hallucination. An estimated 60% of older adults with new psychosis have a medical cause.
• Answer: Medical and neurological workup first. If a cause is confirmed, psychotic disorder due to another medical condition.

Case 7. Two days after hip surgery, an 80-year-old is calm and oriented in the morning. At night she pulls at her IV, doesn't know where she is, and sees bugs on the wall. Her daughter says she was sharp last week.

• Reasoning: Sudden onset, fluctuating attention, and a change from baseline (confirmed by collateral).
• Answer: Delirium, not dementia or schizophrenia. If she also has a prior NCD, the delirium gets treated first.

Case 8. A 35-year-old seeks help for her third depressive episode and denies ever feeling "high." Her husband describes a week last spring when she was unusually upbeat, felt rested on 4 hours of sleep, talked nonstop, and took on several new projects. Coworkers noticed, but she kept working well.

• Answer: Bipolar II disorder. The informant revealed a hypomanic episode: a clear change others noticed, without marked impairment.

Case 9. A 9-year-old has been fidgety, distractible, and impulsive at home and school since kindergarten. His mood shifts several times a day. He has never had days of elevated mood or little need for sleep.

• Answer: ADHD. Same-day mood swings are not hypomania, which needs 4+ days, and bipolar disorder is rare before adolescence. Still ask about trauma and snoring, since PTSD and sleep apnea can mimic ADHD.

Case 10. A woman was fired 6 weeks ago. She cries daily, sleeps poorly, and has stopped looking for work, but she has only three depressive symptoms.

• Answer: Adjustment disorder with depressed mood. The stressor isn't a Criterion A trauma, onset came within 3 months, and she doesn't meet MDD. If she had been assaulted 2 weeks ago and had flashbacks and avoidance, think acute stress disorder; after 1 month, PTSD.

Case 11. A 26-year-old is completely convinced his mother will die unless he touches the doorframe 12 times. Rituals take 3 hours a day. He has no hallucinations, and his speech is organized.

• Answer: OCD, with absent insight/delusional beliefs, not a psychotic disorder.

Case 12. A 15-year-old avoids class presentations and is quiet with new classmates because she fears being judged. With close friends she chats easily and gets jokes and sarcasm. Her early development was normal.

• Answer: Social anxiety disorder. Her skills developed, but fear blocks them. Lifelong weak social communication with no repetitive behaviors would suggest SPCD. Repetitive behaviors now or in the past would make it ASD.

Case 13. A 22-year-old has intense mood shifts lasting hours after fights with her partner, frantic fear of being left, a string of stormy relationships, chronic emptiness, and self-harm since age 14. She has never had a multi-day period of elevated mood or reduced need for sleep.

• Answer: This pattern fits borderline personality disorder, not bipolar disorder. If she presented during an untreated depressive episode, you'd confirm the lifelong pattern before diagnosing BPD.

Diagnostic Overshadowing

Diagnostic overshadowing means blaming new problems on a diagnosis the person already has. Reiss et al. (1982) found that psychologists were less likely to call the same phobia an emotional disturbance when the client was described as having intellectual disability. The effect extended to schizophrenia and personality disorder. It also hits physical health: physical symptoms in people with mental illness get pinned on their psychiatric diagnosis (Hallyburton, 2022). DSM-5-TR notes that people with intellectual disability have some mental and medical conditions at 3 to 4 times the general rate. Many can't describe physical symptoms, so illnesses go undiagnosed (APA, 2022).

Case 14. Staff say a man with moderate intellectual disability is "acting out" with new head-banging and aggression. Collateral shows it began 3 weeks ago, along with poor sleep and refusing food.

• Answer: Don't blame the disability. Refer to rule out pain or medical illness, and assess for depression, which occurs at every level of intellectual disability.

Caveat: a 2024 systematic review found no overshadowing in a third of studies and rated the evidence low quality (Dell'Armo & Tassé, 2024). The Cross-Cultural Issues lesson covers overshadowing in other groups.

Culture: Avoiding Over- and Under-Pathologizing

Breaking a cultural norm is not, by itself, a sign of disorder (Spielman et al., 2020). Bias can push errors in either direction.

Over-pathologizing turns normal cultural experience into a disorder. In some cultures, hearing God's voice is a normal part of religious life. Cultural mistrust can be misread as paranoia. In the United States, African Americans with mood disorders and psychotic features are more often misdiagnosed with schizophrenia (APA, 2022).

Under-pathologizing misses real disorder. Unfamiliar ways of showing distress can lead clinicians to misjudge how severe a problem is. Bias can also swap one label for another: ADHD is identified less often in African American and Latinx youth, possibly because clinician bias mislabels their symptoms as oppositional, which inflates disruptive disorder diagnoses (APA, 2022).

Cultural Concepts of Distress

DSM-5-TR replaced "culture-bound syndrome" with cultural concepts of distress, which come in three types (APA, 2022):

• Cultural idioms of distress: shared ways of talking about suffering, like "nerves."
• Cultural explanations: perceived causes.
• Cultural syndromes: symptom clusters a community recognizes as a pattern.

They rarely match one DSM disorder one-to-one, and they span the full range of severity, including presentations that meet no DSM criteria. DSM-5-TR describes ten examples (APA, 2022):

Ataque de nervios is reported by 7% to 15% of U.S. Latinx adults, and some ataques are normal reactions to acute distress, such as at a funeral (APA, 2022).

Case 15. A Latina woman at her brother's funeral screams, trembles, feels heat rising into her head, and briefly feels unreal. It passes, and she has no other symptoms.

• Answer: An ataque de nervios, here a normal expression of acute grief. Don't diagnose panic disorder or psychosis. The CFI helps you learn what the episode means to her.

The Cultural Formulation Interview

The Cultural Formulation Interview (CFI) is a brief semistructured interview in DSM-5-TR Section III. The core CFI has 16 questions in 4 domains (APA, 2022):

The CFI is person-centered. It asks for the client's own views, which helps avoid stereotyping. You can use it with anyone, at any age, in any setting, not just clients whose background differs from yours. An informant version gathers collateral from family or caregivers. Supplementary modules go deeper, including modules for children and adolescents, older adults, and immigrants and refugees. It helps most when your backgrounds differ, when symptoms don't fit criteria neatly, or when severity is hard to judge (APA, 2022).

The CFI sits beside DSM-5-TR's Outline for Cultural Formulation, which covers five areas: cultural identity, cultural concepts of distress, stressors and cultural sources of vulnerability and resilience, cultural features of the client-clinician relationship, and an overall cultural assessment (APA, 2022).

The CFI is like asking travelers to draw their own map before you hand them yours.

EPPP Traps and Common Misconceptions

Misconception 1: "Normal medical tests mean conversion disorder."

• Reality: FND needs positive signs that the symptom doesn't fit neurological disease. It isn't a diagnosis of exclusion.

Misconception 2: "With red flags in the vignette, pick the best-fitting psychiatric diagnosis."

• Reality: When the question asks what to do first and you see late onset, atypical symptoms, or fluctuating attention, the answer is a medical referral.

Misconception 3: "Hypomania after starting an antidepressant is always medication-induced."

• Reality: If the full syndrome persists past the drug's effects, diagnose bipolar disorder.

Misconception 4: "Delusional OCD beliefs mean a psychotic disorder."

• Reality: Diagnose OCD with absent insight/delusional beliefs.

Misconception 5: "A child with mood swings during the day may have bipolar disorder."

• Reality: Hypomania needs 4+ days. Same-day lability fits ADHD or other conditions better.

Misconception 6: "Each cultural syndrome equals one DSM disorder."

• Reality: The match is rarely one-to-one, and some cultural expressions of distress are not disorders at all.

Memory Aids

• Six steps: "Lies, Pills, Ills, Main, Leftovers, Normal" = feigning, substances and medications, medical condition, main (independent) disorder, adjustment or other specified/unspecified, no disorder.
• Is it medical? Think P-T-T: Plausible mechanism, Timing that matches (the strongest clue), aTypical features.
• Refer when it's "Late, Odd, Seen or Smelled, or Swinging": late onset, atypical symptoms, visual or olfactory hallucinations, fluctuating attention.
• Trauma clock: ASD from 3 days to 1 month; PTSD after 1 month; adjustment starts within 3 months and ends within 6 months after the stressor ends.
• CFI: 16 questions, 4 doors = Define (1-3), Cause (4-10), Coping (11-13), Current help (14-16).
• Deception sort: faking for a prize is malingering; faking for the sick role is factitious; no faking is somatic.

Key Takeaways

• Six steps: rule out feigning, then substances and medications, then medical conditions; identify the independent disorder; consider adjustment or other specified/unspecified; set the boundary with no disorder.
• A medical cause is judged by plausibility, timing (the strongest), and atypical features. Symptoms that last about a month past withdrawal suggest an independent disorder, not a substance-induced one.
• Refer for late onset, atypical symptoms, visual or olfactory hallucinations, or fluctuating attention. Physicians run the labs and imaging.
• Big mimics: thyroid disease, hypoglycemia, pheochromocytoma, B12 deficiency, pancreatic cancer, delirium, TBI, sleep apnea, autoimmune disease, HIV, neurosyphilis, steroids, stimulants, and withdrawal.
• Collateral and records establish hypomania, childhood onset, long-standing patterns, and baseline.
• Know the discriminating feature for each look-alike pair, especially the timing rules for trauma disorders and the psychosis-mood overlap.
• Diagnostic overshadowing blames new problems on an old diagnosis. Check for medical illness and new disorders.
• Culture can push errors both ways. Cultural concepts of distress come in three types, and the CFI has 16 questions in 4 domains.

Now cover the look-alike table and say each discriminating feature out loud. Then sketch your own decision tree for anxiety. Practice like that is what makes differential diagnosis stick.