A diagnosis of attention deficit hyperactivity disorder (ADHD) was associated with an almost 3-year delay in the diagnosis of Autism Spectrum Disorder (ASD) when compared with the age of diagnosis of children diagnosed with ASD alone, a new study reveals.

The study, published online September 14 in Pediatrics, found that children with ASD and ADHD whose ADHD was diagnosed first were 16.7 times more likely to receive their ASD diagnosis after age 6 years (P < .001).

Study authors Amir Miodovnik, MD, MPH, from the Division of Developmental Medicine, Boston Children’s Hospital, and Harvard Medical School, Boston, Massachusetts, and colleagues said the data may suggest a need for greater clinician vigilance. “To avoid potential delays in ASD diagnosis, clinicians should consider ASD in young children presenting with ADHD symptoms,” the authors write.

“The American Academy of Pediatrics (AAP) guidelines endorse universal screening for ASD at 18 and 24 months of age. The [American Academy of Pediatrics] also recommends careful developmental surveillance at every well-child visit, a process that emphasizes eliciting parental concerns, considering family history and observing the child during visits,” the authors explain.

“Despite the push for routine screening and surveillance in primary care, our study of children 2 to 17 years old found that, overall, 39.5% were not diagnosed until 6 years or older, similar to the findings of other large surveillance studies and population surveys.”

The researchers analyzed data from the 2011 to 2012 National Survey of Children’s Health, which asked parents about the age(s) at which their child had received a diagnosis of ADHD and/or ASD. The present study included 1496 children aged 2 to 17 years with a parent-reported current diagnosis of ASD

The investigators also compared children diagnosed with ADHD before they were diagnosed with ASD with children with ASD who were diagnosed with ADHD at the same time or later. In this comparison, the diagnosis of ASD was delayed by 3.2 years (P < .001) for children diagnosed with ADHD first.

The researchers controlled for child sex, race, ethnicity, mother’s education, parent-reported ASD severity, household income, and co-occurring conditions including developmental delay, speech problems, and intellectual disability.

“Our study supports the hypothesis that receiving a diagnosis of ADHD before ASD may delay the diagnosis of ASD, and that this delay persists across age and severity of the ASD. Furthermore, if ADHD is diagnosed first, the ASD diagnosis has a higher probability of occurring late (ie, >6 years of age),” the authors report.

The authors suggest the findings could indicate that children diagnosed with ADHD before ASD may “exhibit unique dimensional traits that could bias clinicians toward an ADHD diagnosis.” Further, they say the results suggest that screening measures may need to take overlapping symptoms of ASD and ADHD into account.

One coauthor has received grants from SynapDx. The other authors have disclosed no relevant financial relationships.

Pediatrics. Published online September 14, 2015.