To date, nearly all research of subtype differences in ADHD has been performed in children and only two studies, with conflicting results, have covered this subject in adults with ADHD.

This study examined subtype differences in the clinical presentation of ADHD-symptoms, related psychopathological features, psychosocial functioning and comorbid psychiatric disorders in adults with ADHD.

One hundred and eighteen adults with ADHD, diagnosed according to DSM-IV criteria, and a population based control group underwent diagnostic evaluations with clinical interviews for ADHD, DSM-IV disorders and demographic features. Comparisons were made between ADHD combined type (n = 64), predominantly inattentive type (n = 30) and predominantly inattentive type, anamnestically combined type (n = 24), relative to each other and to a community control group (n = 70).

The four groups did not differ in age and gender composition. All ADHD groups had significantly less education, were significantly more often unemployed and reported significantly more lifetime psychiatric comorbidity than controls. In comparison to each other, the three ADHD groups differed mainly in core symptoms and the pattern of comorbid psychiatric disorders, whereas no prominent differences in associated psychopathological features and most of the assessed psychosocial functions could be found. Patients with ADHD combined type and inattentive, anamnestically combined type both presented with significantly more hyperactive symptoms and also showed more impulsive symptoms than those with the predominantly inattentive type. With a similar overall lifetime psychiatric comorbidity in the three groups, patients with ADHD combined type and inattentive, anamnestically combined type suffered significantly more from lifetime substance use disorders than patients with predominantly inattentive type.

Our results clearly show impaired psychosocial adjustment and elevated risk for additional psychiatric disorders in adults with all subtypes of ADHD, compared to healthy controls. They provide preliminary evidence that in adult ADHD there might be a subgroup of patients, which is classified as predominantly inattentive subtype according to current diagnostic criteria, but which in its clinical presentation is in between ADHD combined and inattentive type. Further studies are needed to evaluate this finding and to gain a clear picture of its validity.