Differences between early-diagnosed and late-diagnosed ADHD: a follow-up study of former adolescent psychiatric inpatients

Abstract

Objective: The study aimed to investigate whether participants with early- versus late-diagnosed ADHD differed in relation to family, school, sociodemographic, and clinical factors. Method: The study population consisted of 508 adolescents (aged 13–17) hospitalized in a psychiatric unit at Oulu University Hospital between 2001–2006, with follow-up until 2023 via the Finnish Care Register for Healthcare. Participants were grouped as follows: early-diagnosed ADHD (diagnosed before the age of 18) (n = 29), late-diagnosed ADHD (diagnosed after the age of 18) (n = 55), and no ADHD (n = 424). Differences between ADHD groups were assessed using chi-squared, Fisher’s exact, one-way ANOVA, or Kruskal–Wallis tests. Associations were analyzed with logistic regression using late-diagnosed ADHD as the outcome and comparing it separately with the no-ADHD and early-diagnosed ADHD groups. Results: Early-diagnosed ADHD was more prevalent in males, while late-diagnosed ADHD was more common in females. The late-diagnosed ADHD group had more psychiatric comorbidities and was found to report fewer ADHD symptoms in adolescence than the group with no ADHD diagnosis. Female sex combined with anxiety disorders increased the likelihood for late-diagnosed ADHD, whereas conduct disorder was associated with early-diagnosed ADHD. The study also revealed a sharp rise in new ADHD diagnoses during the COVID-19 pandemic in 2021–2022. Conclusion: This study suggests that female-majority late-diagnosed cases may represent a distinct often underdiagnosed clinical group in adolescence. The debate over whether late-diagnosed ADHD is equal to late-onset ADHD and it could reflect a genuine neurodevelopmental variation or arises from comorbidities and life circumstances highlights the need for further longitudinal research.