A differential diagnosis is the process of analyzing a client’s history, test scores, interview data, collateral information, and so on to distinguish between various possible diagnoses—it’s a consideration of possible alternative explanations and trying to rule all of them out except whatever remains as the best explanation. Often the possibilities in question are reasonably similar to each other so the clinician must consider what factors differentiate one particular diagnosis from another.

To illustrate, imagine someone limping into a doctor's office after a soccer game, complaining of foot pain. The doctor probably wouldn’t be considering conditions such as sleep apnea or radiation poisoning initially. Instead, their mind would be occupied with more plausible causes, such as a torn muscle, a strained ligament, or a fractured ankle. The question then becomes: how does the doctor differentiate between these similar yet distinct possibilities? While analyzing, what might catch their attention prompting a consideration of deeper origins, such as bone cancer, genetics, or an autoimmune issue? Could these be related to the foot injury, or is it a simple muscle tear? The process of sifting through these possible diagnoses, considering the likelihood of each based on the available information, is what constitutes a differential diagnosis. In most scenarios with the foot injury, the conclusion will be straightforward, but the process demands deep knowledge in relevant areas, and complex cases show up often enough that every situation necessitates a thoughtful approach.

A differential diagnosis is of particular importance when assessing autism, especially in adults. There is an incredible amount of complex overlap between autistic experience and a variety of other concerns such as complex trauma, ADHD, social anxiety, avoidant personality disorder, developmental coordination disorder, depression, burnout, schizotypal personality disorder, and so on.

To further complicate matters, autistic people often have experienced trauma, almost always have at least some executive functioning differences (which may or may not meet full criteria for ADHD), are often burned out, frequently experience mild to substantial amounts of anxiety and distress due to social masking, disrupted routines, being socially overwhelmed, and sensory sensitivities; and so on.

To demonstrate how diverse these combinations can be, a 2019 meta-analysis showed significant rates of the following co-occurring diagnoses in autistic people:

• 28% had ADHD

• 20% had an anxiety disorder

• 13% had sleep-wake disorders

• 12% disruptive, impulse-control, and conduct disorders

• 12% had depressive disorders

• 9% had OCD

• 5% had bipolar disorder

• 4% had schizophrenia spectrum disorders

A 2018 meta-analysis reported that approximately 50% of autistic people have alexithymia, which is when you have difficulty noticing and articulating your emotional experience. Prevalence of alexithymia in a ‘normal’ population is approximately 5%. Various tic disorders are much more common for autistic people, as is epilepsy, and many other conditions.

Accurate and comprehensive differential diagnosis is crucial for understanding each individual’s unique psychological landscape, especially when assessing for autism in adults. It's paramount to carefully sift through each layer to ensure that we are not misinterpreting one experience and its origins for another. This isn’t some academic exercise; it’s a practical necessity to offer the most effective support for each individual, allowing them to navigate their world with a greater sense of understanding and acceptance. The complexities involved in this diagnostic process underscore the importance of a thoughtful, well-informed, and empathetic approach, one that honours the individuality of each person's experiences and challenges.