Why Do Adults Get Misdiagnosed With ADHD (And What Actually Shows Up in Testing)

Author: Dr. Larry Brooks, Ph.D. | Board-Certified Neuropsychologist

Educational Disclaimer: This article is for educational and informational purposes only and is not a substitute for professional medical advice, neuropsychological diagnosis, or treatment. Accessing this content does not establish a doctor-patient relationship.

Adult ADHD diagnoses have more than tripled in the past two decades. More people are getting answers — but more people are also getting the wrong ones. If your treatment hasn’t worked, the first question to ask is whether your diagnosis was correct.

Reading time: 14 minutes

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Key Takeaways

• Over 15.5 million U.S. adults now have an ADHD diagnosis — and nearly 56% of them were first diagnosed as adults

• 75% of adults with ADHD have at least one other psychiatric diagnosis — making accurate diagnosis hard without objective testing

• Anxiety, depression, sleep disorders, early bipolar disorder, and post-concussion syndrome all mimic ADHD symptoms

• A self-report checklist is not the same as neuropsychological testing

• Neuropsychological testing measures actual brain function: attention, memory, processing speed, and response control, with objective data

• If treatment hasn’t helped, a comprehensive evaluation may show why

Is adult ADHD really that hard to diagnose?

Yes. More than most people realize.

Concentration problems are the second most commonly reported symptom across the entire DSM-5, the diagnostic manual clinicians use to classify mental disorders. That means difficulty focusing isn’t unique to ADHD. It shows up in depression, anxiety, sleep disorders, trauma, early bipolar disorder, and the aftermath of a concussion. It shows up in grief. It shows up when someone is under extreme stress.

When a clinician asks, “Do you have trouble staying focused?” and the answer is yes, that single piece of information tells you very little on its own.

The problem is that many adult ADHD diagnoses are built almost entirely on that kind of self-reported information. A questionnaire, a clinical interview, maybe a behavioral rating scale. The patient describes symptoms. The clinician matches those symptoms to a checklist. A prescription follows.

That process works some of the time. For straightforward presentations in patients with clear childhood histories, it can be sufficient. But for adults, where ADHD symptoms are subtler, where comorbid conditions are common, and where the history is often incomplete, it leaves a lot of room for error.

What conditions get misdiagnosed as ADHD in adults?

This is where it gets complicated:

Several psychiatric conditions produce symptoms that look nearly identical to ADHD on a questionnaire.

Anxiety disorders.

Poor concentration, restlessness, difficulty completing tasks, racing thoughts. Every one of those is a hallmark ADHD symptom. Every one of them also describes an anxious adult under chronic stress. Research published in found that more than 56% of adults with ADHD also meet criteria for at least one anxiety disorder.

When both are present, sorting out which symptoms belong to which condition requires more than a checklist.

Depression

Cognitive slowing, loss of focus, poor memory, and difficulty initiating tasks are symptoms of depression, not just ADHD. A person who has been depressed for years can present to a clinician looking very much like someone with ADHD. If the depression isn’t identified as the primary driver, stimulant medication won’t work. The underlying condition is left untreated.

Bipolar disorder

This one is underappreciated. According to research, bipolar disorder and ADHD have significant overlap in their symptom profiles and high rates of co-occurrence. Impulsivity, mood shifts, difficulty sustaining attention, and sleep disruption appear in both.

Bipolar II, in particular, is frequently missed because the hypomanic episodes are less obvious.

The result: patients get diagnosed with ADHD when the real issue is an undetected mood disorder, and stimulants, the standard ADHD treatment, can make bipolar symptoms worse.

Sleep disorders

Untreated sleep apnea produces cognitive symptoms that are functionally indistinguishable from ADHD. Fatigue, poor working memory, difficulty concentrating, and mood instability. A person who has never had a proper sleep evaluation may spend years being treated for ADHD when the root cause is a condition that has nothing to do with attention.

Post-concussion syndrome

Cognitive effects of a past head injury, even a mild one, can surface as attention problems, processing speed deficits, memory complaints, and emotional dysregulation. Without a thorough history and objective cognitive testing, the brain injury connection often gets missed entirely.

Why do so many ADHD diagnoses happen without neuropsychological testing?

A few reasons:

First, a formal neuropsychological evaluation takes time, typically four to eight hours of testing, plus interpretation, collateral interviews, and a written report. It is thorough by design. That’s not how most clinical encounters are structured.

Second, many providers who diagnose ADHD were not trained in the specific patterns that differentiate it from other conditions. Concentration problems are common. If the first explanation that comes to mind is ADHD, and the questionnaire supports it, that often becomes the diagnosis.

Third, the diagnostic criteria for ADHD in the DSM-5 are based largely on childhood presentations. Rambunctious kids are easy to identify. Adults with ADHD don’t climb on desks at work. They miss deadlines. They lose things. They feel perpetually behind. Those symptoms are real, but they also describe dozens of other conditions.

A 2024 report from the National Academies of Sciences, Engineering, and Medicine noted that “ADHD has a childhood face” and that clinicians often apply childhood-based criteria to adult presentations in ways that produce diagnostic errors.

What does neuropsychological testing actually measure?

Here is the core difference:

Neuropsychological testing doesn’t ask you how you feel. It measures how your brain performs — on standardized tasks, under controlled conditions, compared to large normative samples of people your age.

A comprehensive evaluation examines:

Working memory. Can you hold information in mind and manipulate it? This is the cognitive function most consistently impaired in genuine ADHD. It’s also affected by anxiety, sleep deprivation, and depression — but the patterns look different.

Sustained attention. Can you maintain focus over time? How does your performance degrade as a task continues? This is measured, not estimated.

Processing speed. How quickly does your brain take in and respond to information? Significant slowing can indicate depression, post-concussion effects, or other neurological factors that don’t respond to stimulants.

Response inhibition: The ability to stop an automatic response. This is where impulsive errors show up in the data. It separates true attentional impulsivity from the hyperreactivity that characterizes anxiety or bipolar disorder.

Executive function: Planning, organization, cognitive flexibility, problem-solving. The full picture of frontal lobe function — not a single score, but a profile.

A good evaluation also includes built-in validity measures: tests within the tests that detect whether the results are accurate. This matters. It protects against the over-diagnosis of impairment and provides defensible data for clinical, academic, or legal purposes.

Does someone with real ADHD need neuropsychological testing to be diagnosed?

Not always.

For clear, uncomplicated presentations with a solid developmental history and no significant comorbidities, a clinical interview and behavioral rating scales may be sufficient.

But that describes a minority of adults who seek evaluation.

Most adults who end up pursuing ADHD testing have already tried something that didn’t work. They’ve been through treatment, sometimes years of it, without meaningful improvement. The medication helped a little, or not at all, or caused problems. The therapy addressed anxiety, but the concentration issues remained. They’ve been told they have ADHD, or that they don’t, and they’re still not sure.

That’s the patient who genuinely benefits from objective testing. Not because testing is required for a diagnosis, but because at that point in the diagnostic journey, guesswork has already failed them.

What does the dual-diagnosis problem mean in practice?

Research consistently shows that ADHD rarely travels alone. A 2021 publication in BJPsych Advances noted that 75% of adults with ADHD have at least one other psychiatric diagnosis, and that it is frequently the comorbid condition that gets identified while the ADHD itself is missed. The reverse is also true: a person gets treated for ADHD while the depression or anxiety driving their symptoms goes unaddressed.

This is where clinical breadth matters. A provider who has evaluated patients primarily in one setting, such as outpatient treatment for anxiety and depression, for example, may not have the clinical pattern recognition to see when ADHD and a mood disorder are both present, or when a presenting complaint that looks like ADHD is actually something else entirely.

The goal of a proper evaluation isn’t to confirm a suspicion. It’s to build an accurate picture of what’s actually happening, and, where multiple things are happening simultaneously, to identify all of them.

What does a thorough ADHD evaluation look like for adults?

A comprehensive adult evaluation includes several components that self-report alone can’t provide.

A full developmental and psychiatric history.

When did symptoms start? What was childhood like, academically and behaviorally? What other treatments have been tried? What happened when they were tried?

Collateral history

Input from someone who knows the patient well, a spouse, family member, or close colleague, adds a dimension that self-report misses. Patients are often not the most accurate observers of their own symptoms.

A battery of standardized cognitive tests

Not a single scale, but multiple measures across attention, memory, processing speed, executive function, and cognitive flexibility.

Validity testing

Embedded measures throughout the evaluation that assess whether performance reflects genuine ability or is skewed by fatigue, effort, or secondary gain.

Clinical interpretation. A trained neuropsychologist who can read the patterns — who understands when a processing speed deficit points to depression versus post-concussion syndrome, and when an attention profile fits genuine ADHD versus anxious distraction.

The output is a written report that ties everything together: what the data shows, what it means clinically, and what should happen next.

How do I know if I need neuropsychological testing for ADHD?

Ask yourself a few questions.

• Have you been treated for ADHD, or for anxiety, or depression, and the treatment hasn’t worked as expected?

• Has your diagnosis changed more than once?

• Do you have a significant history that might complicate the picture, like a past head injury, a period of heavy substance use, or a family history of mood disorders?

• Are you facing a decision where an accurate diagnosis matters, accommodations, legal proceedings, or medication choices?

If the answer to any of those is yes, objective neuropsychological testing gives you information that no questionnaire can provide.

A proper evaluation isn’t a longer version of the same process you’ve already been through. It measures function directly. It catches what clinical interviews miss. And it produces a report that follows you into treatment decisions, into educational accommodations, into the courtroom if it comes to that.

"Why Do Adults Get Misdiagnosed With ADHD?"

Here's the bottom line:

The surge in adult ADHD diagnoses reflects something real: adults who spent decades without answers are finally getting them. But the same environment that has made diagnosis more accessible has also made misdiagnosis more common.

Concentration problems are not a diagnosis. They are a symptom. Tracking down what’s actually causing them, whether it’s ADHD, anxiety, depression, a mood disorder, residual post-concussion effects, or several of these at once, requires more than a checklist.

Neuropsychological testing assesses the brain, not only the symptom reports. For adults with complex histories and treatment courses that haven’t worked, that distinction is the difference between more of the same and an actual answer.

Schedule a consultation with Dr. Brooks

Dr. Larry Brooks is a board-certified clinical neuropsychologist based in Hollywood, Florida. He conducts comprehensive neuropsychological and psychological evaluations for adults and adolescents across South Florida, including Fort Lauderdale, Miami, Pembroke Pines, and Boca Raton. He is the consulting neuropsychologist for the Florida Panthers and a Qualified BAP Provider in the NFL Concussion Settlement Program.

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Sources:

- CDC. (2023). Data and Statistics on ADHD.

- Kooij et al. (2021). Misdiagnosis and missed diagnosis of adult attention-deficit hyperactivity disorder. BJPsych Advances.

- Comparelli et al. (2022). Differentiation and comorbidity of bipolar disorder and ADHD. Frontiers in Psychiatry.

- Choi et al. (2022). The prevalence of psychiatric comorbidities in adult ADHD. PLoS One.

- National Academies of Sciences, Engineering, and Medicine. (2024). Impact of Misdiagnosis, Bias, and Stigma — Adult ADHD.

-Adult ADHD: Overdiagnosed? Or finally getting the attention it deserves?

- Rivas-Vazquez et al. (2023). Adult ADHD: Underdiagnosis of a Treatable Condition. Journal of Health Service Psychology