To Örebro University

Background: The diagnosis of attention-deficit/hyperactivity disorder (ADHD) in preschool years (before age 6 years) is a marker of severity and poor prognosis. This study investigated a broad range of predictors of ADHD diagnosis during preschool age.

Methods: Population-based cohort study using Swedish registers. The final sample consisted of all children born in Sweden between 2001 and 2007 who could be linked to both of their biological parents, excluding those who died or emigrated (n = 631,695). Follow-up was completed December 31, 2013. Cox proportional-hazards models for survival analysis were used to identify the predictors that increased the risk of receiving a clinical diagnosis of ADHD from 3 to 5 years. Hazard ratios (HR) with 95% confidence intervals (CI) were presented for each of the 41 selected predictors covering early-onset psychiatric comorbidities, nonpsychiatric medical conditions, parental history and perinatal factors.

Results: At the end of follow-up, 1,686 preschoolers (2.7% of the whole sample) had received a diagnosis of ADHD. We found that 39 out of 41 predictors were associated with increased risk of a later diagnosis of preschool ADHD. Novel associations with preschool ADHD diagnosis were found for gastroesophageal reflux disease (HR = 3.48), premature contractions during pregnancy (HR = 2.03), and criminal conviction history from any parent (HR = 2.14).

Conclusions: A large number of novel and well-established predictors of preschool ADHD diagnosis were identified. This broad set of early predictors may direct future clinical research and assist in early identification of preschool ADHD.