National survey results show as many as 1 in 40 U.S. children have been diagnosed with autism, continuing an upward trend.

Researchers estimate 1.5 million American children ages 3 to 17 have been diagnosed with the developmental disorder, for a prevalence rate of 2.5 percent. The figures published online Monday in the journal Pediatrics come from data collected through the 2016 National Survey of Children’s Health, a government survey of parents of more than 50,000 children across the country.

As part of the survey, parents were asked if a doctor or other health care provider had ever told them that their son or daughter had autism and, if so, they were asked if the child currently has the condition.

“It is difficult to pin down an exact number. We don’t have a biological marker for autism,” said Michael Kogan of the federal Health Resources and Services Administration and lead author of the new report. “We know that in terms of having a major condition like autism, parents are usually pretty good reporters compared to medical records.”

While the government tracks autism prevalence through several methods including the parent survey, the official prevalence rate is based on data collected through the Centers for Disease Control and Prevention’s Autism and Developmental Disabilities Monitoring Network.

In April, the CDC reported from that data collection that autism affects an estimated 1 in 59 children based on medical and educational records of thousands of 8-year-olds at multiple sites in the network.

While there is some debate over whether parent surveys or medical records are more accurate, the true prevalence rate is probably somewhere between 1 in 40 and 1 in 59, said Alycia Halladay, chief science officer at the Autism Science Foundation.

“There is evidence to say that the method of collection impacts prevalence numbers,” Halladay said.

The autism prevalence rate of 2.5 percent found in the 2016 parent survey is up from 1.7 percent in 2014 and 1.5 percent in 2012. But Kogan cautioned that the rise could be mostly attributable to changes in the survey format.

Changes in the diagnostic method and better awareness among clinicians are thought to contribute to the rise in autism rates over the past few decades. But there is also an unknown percentage that could represent a true rise in cases, Halladay said.

“There is a part that we don’t know where it’s coming from,” she said. “There probably is a real increase but we don’t know what’s causing it yet.”

The study also provides the first national estimates on the prevalence of treatment for autism, Kogan said. About 64 percent of parents who indicated that their child had autism reported that their son or daughter had received behavioral therapy in the last year, and 27 percent reported that their child had been treated with medication for autism symptoms including irritability.

The survey results also show the challenges parents face in getting appropriate care, from referrals to specialists to finding family-centered care.

“Parents of children who have autism spectrum disorder are quite frustrated with trying to get services for their children, even compared to kids with other emotional, behavioral and developmental disorders,” Kogan said.

There is other evidence that children and adults with autism have problems accessing care for psychiatric and other medical issues, Halladay indicated.

“Clinicians and the community should take this with other data and say we need to do a better job at making sure we’re helping everyone in the family with autism in addition to any other issues that they may have,” she said.

The research is published in the December 2018 edition of the journal Pediatrics.