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54 Rohde, Buitelaar, Gerlach & Faraone betes. The exact mechanisms explaining these comorbid profiles is not yet un- derstood but might be related to general and chronic immune and inflammatory dysregulations. 22 Clinically, some issues need to be highlighted: 1 the comorbidity with ODD is by far the most common in samples of children and adolescents. Thus, clinical investigation of ODD is mandatory when fa- cing a positive ADHD diagnosis; 2 the presence of some comorbidities like conduct disorder increases the chance of other sequential comorbidities like SUDs. Thus, clinicians asses- sing adolescents with ADHD comorbid with conduct disorder should give special attention to the assessment of SUDs; 3 some comorbid disorders might reflect a co-occurring diagnosis alongside ADHD in some situations (e.g., SUDs, Generalized Anxiety Disorder – GAD, Depression), or a differential diagnosis in others (e.g., when an adult with Recurrent Major Depression only presents significant inattentive symp- toms and executive functioning deficits during the active phase of the mood disorder). Thus, clinicians should assess carefully if the symptoms of the as- sociated disorder explain the ADHD phenotype or co-occur and interact with the ADHD phenotype, making the final phenotype even more com- plex. In these situations, it is important to ask patients if the main ADHD symptoms occur only in the presence of the symptoms of the co-occurring disorder, or independently of them. For example, it might be clinically rele- vant in a patient with ADHD and GAD symptoms to try to characterize if the difficulty paying attention in class or at work is related only to worries and dysfunctional thoughts associated to performance or inattention also occurs in moments without anxiety and tension being related to neutral or agreeable thoughts. ANCILLARY DIAGNOSTIC APPROACHES As for all other psychiatric conditions, there is no ancillary test or biomarkers with sufficient positive and negative predictive power for the diagnosis of ADHD. 2,4,21 Some tests can be relevant and valuable to depict an individual’s cognitive strengths and weaknesses, but these do not need to be performed routinely. In cases where there are questions about a young person’s intellectual impairment, potential learning disorders, or severe executive functioning deficits, additional neuropsychological testing may be warranted. Either a full intelligence test or – when time and resources are scarce – a shortened version should be applied when there are questions about learning progress or classroom adjustment. 18