Patients with an at-risk mental state (ARMS) for psychosis and patients with attention deficit hyperactivity disorder (ADHD) have many overlapping symptoms and hence can be difficult to differentiate clinically.

The aim of this study was to investigate whether the differential diagnosis between ARMS and ADHD could be improved by neuropsychological testing.

A total of 157 ARMS and 122 adult ADHD patients were recruited via the Basel Früherkennung von Psychosen (FePsy) study and the ADHD Special Consultations Unit of the University of Basel Psychiatric Hospital, respectively. Verbal learning and memory was tested with the California Verbal Learning Test (CVLT), sustained attention with the Continuous performance test (CPT) and problem solving abilities with the Tower of Hanoi task. Group differences in neuropsychological performance were analyzed using generalized linear models, which included age and gender as covariates.

Adult ADHD patients recalled significantly fewer words in the CVLT (both after short and long delay) and had significantly more false alarms and omissions and longer reaction times in the CPT than ARMS patients.

Adult ADHD patients show larger deficits than ARMS patients in the domains of verbal memory and sustained attention, but not in problem solving abilities. This in line with current meta-analyses, which found that impairments in the domains of attention and verbal memory are of medium effect size in adult ADHD patients and of small effect size in ARMS patients. Our results suggest that measures of these domains can be exploited to improve the differential diagnosis between adult ADHD and ARMS patients.