The latest government data shows that autism prevalence is on the rise, but a newly-adopted standard for diagnosing the developmental disorder may be muddying the numbers.

The definition of autism got a major overhaul in 2013 when the American Psychiatric Association adopted a new edition of its Diagnostic and Statistical Manual of Mental Disorders, known as DSM-5.

Under the updated manual, Asperger’s syndrome, childhood disintegrative disorder and pervasive developmental disorder, not otherwise specified were folded under a broader diagnosis of autism spectrum disorder, with clinicians indicating a level of severity. The revised criteria for autism — which some consider more restrictive — require both deficits in social communication and social interaction as well as restricted repetitive behaviors, interests and activities for a diagnosis. However, it also includes anyone who had a previous diagnosis of autism.

Now, for the first time, the Centers for Disease Control and Prevention is factoring the changing definition of autism into its regular tracking of autism prevalence.

Figures released in April show that 1 in 59 children have autism — an increase from reports two years ago that found 1 in 68 children were on the spectrum. However, the latest numbers, which rely on data collected on over 300,000 children who were 8 years old in 2014, are based on the older definition of autism.

In the same report, the CDC then evaluated the children under the newer DSM-5 criteria, finding that 18 percent fewer would qualify for an autism diagnosis. Using the updated definition and also including children who had previously received a diagnosis of autism, 4 percent fewer children were found to have autism.

The goal of doing separate analyses using both definitions was to “really give a direct comparison of the difference in prevalence and characteristics based on those two case definitions,” said Daisy Christensen, an epidemiologist on the surveillance team for the developmental disabilities branch of the CDC.

Going forward, the DSM-5 definition will be the primary criteria used by the CDC’s Autism and Developmental Disabilities Monitoring Network for its biennial tracking of children with autism, though the research will also include those diagnosed under the previous definition, Christensen said. The CDC will release its next evaluation, based on 2016 data, in two years.

While it’s possible that the new criteria for autism diagnosis “might exclude some” children who met only the older definition, it’s more likely that any trends of increase or decrease in prevalence will be influenced more by clinicians adjusting their evaluations to the new definition and other factors, she said.

Tom Frazier, chief science officer for the advocacy organization Autism Speaks, said that the impact of the new definition on autism rates will take years to play out.

“This analysis suggests that it is possible that DSM-5 will result in fewer cases being identified, and this may make it difficult to discern whether autism prevalence continues to increase,” Frazier said of the CDC report.

Clinicians themselves need to get used to the criteria and applying them, he said. The data will be in a “transitional phase” until DSM-5 is the standard being used universally, Frazier said.

The biggest hope, he said, is that the new definition will provide “a much more solid basis for saying someone has autism. It won’t be a blood test, but will still be much more objective and quantitative.”

However, Catherine Lord, director of the Center for Autism and the Developing Brain at Cornell University and a professor at Weill Cornell Medical College, who was a member of the committee that crafted the DSM-5 definition, said she doesn’t expect the new criteria to significantly impact the prevalence data in the future.

The intention behind the new definition “was to make things more straightforward and clearer, so there’s not as much confusion” over separate diagnoses available under the older version.

In fact, Lord said she expects rates of autism to continue to rise, in part because minority communities are becoming more informed about the disorder and minority children are being diagnosed more often.

“You always need to be concerned about missing people,” she said, “but there is room for everybody who needs to be there in that new criteria.”

Read more stories like this one. Sign up for Disability Scoop's free email newsletter to get the latest developmental disability news sent straight to your inbox.