A consistent diagnosis and monitoring process is needed for accurate diagnosis of ADHD and co-occurring diagnoses in the Australian context. It is important that clinicians are aware of which conditions commonly co-occur with ADHD (see section 1.1), as the presence of a co-occurring condition may result in a missed diagnosis of ADHD or a missed diagnosis of a co-occurring condition when ADHD symptoms are present.
A high proportion of people with ADHD have co-occurring neurodevelopmental, mental health and medical conditions (Background section; High-risk groups section 1.1). ADHD can be diagnosed in the presence of other conditions.
In children, the most common co-occurring disorders are oppositional defiant disorder, language disorders, autism spectrum disorders and anxiety disorders, with depressive disorders and substance use disorders emerging in adolescence. Specific learning disorders also commonly occur in people with ADHD and involve difficulties in reading, written expression or mathematics (DuPaul, Gormley, & Laracy, 2013). Among adults with ADHD, the most common co-occurring mental health disorders are depressive disorders, bipolar disorders, anxiety disorders, personality disorders and substance use disorders (Kessler et al., 2006).
Medical conditions, such as epilepsy, acquired brain injury, and foetal alcohol spectrum disorder, can co-occur with ADHD (Ilie et al., 2015; Lange, Rehm, et al., 2018). For people with ADHD and a co-occurring condition, the onset, duration and pattern of functional impact can help differentiate the effects of ADHD from those of the other conditions to help guide the treatment plan.
Differential diagnosis involves differentiating between two conditions that share similar symptoms. Several medical disorders can be present and have symptoms and signs similar to those of ADHD. For example, sleep disorders (Baddam et al., 2021), hearing or vision impairment, thyroid disease (American Psychiatric Association, 2013) and anaemia (Konofal, Lecendreux, Arnulf, & Mouren, 2004). Several medications can also produce symptoms similar to those of ADHD (American Psychiatric Association, 2013).
Clinicians should conduct a comprehensive assessment (including history and examination) to identify any possible differential medical causes for ADHD. The majority of people with ADHD do not need laboratory investigations as part of their differential diagnostic assessment. In some circumstances, specific laboratory tests may be needed to exclude a suspected medical cause of ADHD symptoms.
In addition to medical conditions, neurodevelopmental and mental health conditions should be considered during differential diagnosis. This is due to their high level of co-occurrence and needs to be identified and treated, or they may be differential diagnoses potentially misdiagnosed as ADHD (American Psychiatric Association, 2013).
Given the symptoms of ADHD may overlap with symptoms of other related conditions, careful consideration of the onset and course of symptoms is required to make decisions about differential diagnosis. For example, difficulties with concentration and focusing attention that are associated with a major depressive episode are usually limited in duration, whereas attention problems due to ADHD are lifelong. For each condition that may be a differential diagnosis with ADHD, consider the overlapping symptoms and those that are distinct to the differential condition.
As noted in section 2.1, the use of broad band rating scales may assist in identifying possible differential conditions. Narrow-band rating scales for identified possible differential diagnoses may assist in providing further clarification. Neuropsychological evaluation may also assist with differential and co-occurring condition diagnosis when there is diagnostic uncertainty, as noted in section 2.1. Best-practice guidelines for the diagnosis of the identified differential or co-occurring conditions should be consulted.
There are no specific conditions that must be excluded for a diagnosis of ADHD. DSM-5 provides further specific advice on differential and co-occurring diagnoses (American Psychiatric Association, 2013).
These recommendations are consistent with the existing practice of conducting differential diagnostic assessments for other conditions. While training in differential and co-occurring diagnosis is usual practice for those involved in the diagnosis of neurodevelopmental and mental health conditions, specific information on ADHD should be covered by training, as recommended above. It is noted there is a lack of research on co-occurring conditions in particular subgroups, including Aboriginal and Torres Strait Islander peoples, see section 6.2. Feasibility may be impacted by time and funding constraints that may prevent clinicians from conducting thorough diagnostic assessments.
