Background: A diagnosis of attention-deficit/hyperactivity disorder (ADHD) requires the presence of impairment alongside symptoms above a specific frequency and severity threshold. However, the question of whether that symptom threshold represents anything more than an arbitrary cutoff on a continuum of impairment requires further empirical study. Therefore, we present the first study investigating if the relationship between ADHD symptom severity and functional impairment is nonlinear in a way that suggests a discrete, nonarbitrary symptom level threshold associated with a marked step increase in impairment. Methods: Parent reports on the ADHD-Rating Scale (ADHD-RS-IV), the Weiss Functional Impairment Rating Scale (WFIRS-P), and the Strengths and Difficulties Questionnaire were collected in a general population sample of 1st, 2nd, and 3rd graders (N = 1,914-2,044). Results: Piecewise linear regression analyses and nonlinear regression modeling both demonstrated that the relationship between symptom severity (ADHD-RS-IV total score) and impairment (WFIRS-P mean score) was characterized by a gradual linear increase in impairment with higher symptom severity and no apparent step increase or changing rate of increase in impairment at a certain high ADHD-RS-IV total score level. Controlling for socioeconomic status, sex, and co-occurring conduct and emotional symptoms did not alter these results, though comorbid symptoms had a significant effect on impairment. Conclusions: There was no clear evidence for a discrete, nonarbitrary symptom severity threshold with regard to impairment. The results highlight the continued need to consider both symptoms and impairment in the diagnosis of ADHD.

The literature on subclinical autism spectrum (ASD) symptoms in pediatric obsessive-compulsive disorder (OCD) is scarce, and it remains unclear whether ASD symptoms are related to OCD severity. The aims of the present study were to assess the prevalence of ASD symptoms and age and sex differences in children and adolescents with OCD, and to explore the relation between ASD symptoms and OCD severity. This is the largest study of ASD symptoms in an OCD population to date, and the first directly aimed at elucidating sex and age differences in this matter. The study used baseline data from the Nordic Long-term OCD Treatment Study in which parents of 257 children and adolescents with OCD aged 7-17 completed the Autism Spectrum Screening Questionnaire. OCD severity was assessed with the Children's Yale-Brown Obsessive Compulsive Scale. Pediatric OCD patients were found to exhibit elevated rates of ASD symptoms compared to a norm group of school-age children. ASD symptoms were concentrated in a subgroup with a prevalence of 10-17 %. This subgroup was characterized by a male preponderance with a sex ratio of approximately 2.6:1, while children versus adolescents with OCD exhibited similar rates. Autism-specific social and communication difficulties were not related to OCD severity, while restricted repetitive behavior was positively related to OCD severity. The results indicate that clinicians need to be aware of ASD symptoms in children and adolescents with OCD since one out of ten exhibits such symptoms at a clinical sub-threshold.

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