ADHD Diagnosis Patterns: Comorbidities and the Impact of Relative Age on Diagnosis Likelihood

BACKGROUND: Attention deficit hyperactivity disorder (ADHD) has been associated with comorbid conditions, especially psychiatric comorbidities. On the other hand, getting an ADHD diagnosis has been linked to children's relative age. The aim of this study was to describe diagnostic patterns of ADHD patients of all ages, and to compare comorbidity and birth month between ADHD patients and matched controls. METHODS: This nationwide study cohort included incident ADHD patients with records of ADHD medication and/or ADHD diagnosis in Finland during 2015 to 2020, and with no such records during 2010 to 2014. Four controls were matched with each patient based on age, gender, and place of residence. RESULTS: Altogether 66,146 incident ADHD patients and 256,270 matched controls were identified, and their average follow-up time was 3.5 years. Mean age at the first ADHD diagnosis was 15.9 years for males and 22.3 for females. Prior to being diagnosed with ADHD, patients visited health care remarkably more often than controls. Comorbid diagnoses were the most prevalent at index date, and compared to controls, the prevalence of these comorbidities within 3 years preceding the index date increased more in ADHD patients. The odds of ADHD increased with later birth months across all age-gender groups, except among girls under 6 years of age. The association was more pronounced among males. CONCLUSION: ADHD is associated with a significant comorbidity burden across all age groups, even before ADHD diagnosis. Individuals who are younger in relative age are more likely to be diagnosed with ADHD.