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Many children with autism show early signs of the disorder, such as lack of eye contact and repetitive behaviors. A pediatrician’s ability to spot these subtle signs during a routine exam opens the door to early intervention.

But many toddlers with autism display so few abnormal behaviors during brief exams that they evade the diagnostic eye of even expert clinicians, according to a study published 12 January in Pediatrics. This suggests that behavioral observation alone is not enough for doctors to detect the disorder in young children.

The researchers recorded two 10-minute videos for each of 42 children as they underwent thorough diagnostic exams for autism. The study included 14 children with autism, 14 children with language delay and 14 controls, all between 15 and 33 months of age.

Two autism experts, both of whom were unaware of the children’s diagnoses, reviewed the videos. They measured the number of times each child performed certain behaviors, such as playing with toys, making sounds or responding to their name being called, and rated each behavior as ‘typical’ or ‘atypical.’ They then decided whether each child’s overall behavior warranted a referral for an autism evaluation.

Based on these observations, the autism experts incorrectly flagged 39 percent of the children with autism as not needing further evaluation. That may be because they thought these children showed typical behavior 89 percent of the time. In other words, the atypical behaviors occur so seldom that they are overshadowed by typical ones, the researchers suggest.

This doesn’t necessarily mean that pediatricians and family physicians are missing children with autism, as mainstream news reports suggested last week. Beyond making behavioral observations, doctors typically consider a child’s family history, the results of autism screening tests and parental reports of symptoms displayed outside the doctor’s office.

The findings highlight just how difficult it can be to diagnose autism based on behavior. When rating typical behaviors, the experts in the study agreed 97 percent of the time, but this fell to 35 percent for atypical behaviors. If anything, the study underscores the need for more objective measures of autism symptoms.