Symptoms of attention deficit hyperactivity disorder may mask signs of autism in young children, researchers say, often delaying diagnosis for years.

Among children with both autism and ADHD, autism diagnosis was delayed by an average of three years in kids flagged with ADHD first, according to findings published online this month in the journal Pediatrics.

For the study, researchers looked at information on nearly 1,500 kids with autism collected from their parents through the 2011–2012 National Survey of Children’s Health. Just under half of the children were also diagnosed with ADHD.

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Children first thought to have ADHD alone were 30 times more likely than other kids with both conditions to receive an autism diagnosis after age 6, the study found.

While autism and ADHD can present with similar symptoms — like inattention, hyperactivity and impulsivity — the conditions are unique and clinicians should consider autism when evaluating children for ADHD, researchers said.

“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,” wrote researchers from Boston Children’s Hospital and Harvard Medical School in their findings. “ASD that goes unrecognized and untreated until the child is older may negatively affect their long-term prognosis.”