New guidelines for defining autism spectrum disorders may reduce the number of diagnoses by almost one-third, according to new research from Columbia University.

A review of relevant literature and data, published in the Journal of Autism and Developmental Disorders, raises questions about the future of children who have developmental delays, but no longer meet criteria for an autism diagnosis.

Kristine M. Kulage, who led the review, said the new guidelines, issued last year by the American Psychiatric Association, may leave thousands of children in the U.S. without an autism diagnosis needed to qualify for medical benefits and social services.

The systematic review by Kulage’s team at the Columbia University School of Nursing found a 31 per cent decrease in autism spectrum disorder diagnoses using the new version of the Diagnostic and Statistical Manual of Mental Disorders, compared to the number of cases that would have been diagnosed under the previous version.

DSM, the so-called “bible of psychiatry,” is widely used by experts in both U.S. and Canada to diagnose and classify mental health disorders.

The changes introduced in 2013 affect not only the definition of autism spectrum disorders, but a number of other conditions, including types of obsessive-compulsive disorder and depression triggered by the loss of a loved one.

The old version of the manual, DSM-IV-TR, divided the broad definition of autism spectrum disorder into three subgroups: autistic disorder, Asperger’s disorder and pervasive development disorder.

The new version, DSM-5, has eliminated those subgroups and introduced a more limited range of criteria for autism spectrum disorders. The term Asperger’s syndrome has been dropped.

A new category, called social communication disorder, or SCD, was added to help diagnose children who have problems communicating both verbally and non-verbally, but don’t exhibit other symptoms associated with autism.

"This study raises a concern that a medical provider diagnosing a child under the new guidelines won't find the child to be on the autism spectrum, when the same child under the old criteria might have been diagnosed with ASD," Kulage said in a news release.

"We are potentially going to lose diagnosis and treatment for some of the most vulnerable kids who have developmental delays," Kulage said. "In many instances, children require a diagnosis of ASD to receive medical benefits, educational support and social services."

Under DSM-5, there was a decrease in autistic disorder diagnoses of 22 percent, compared with the previous edition of the manual, the Columbia review found. There was also a decrease of 70 percent in diagnosis of pervasive development disorder.

Asperger's diagnoses also declined, but the rate was not statistically significant, the review found.

When proposed changes to the DSM were floated in 2012, many Canadian parents expressed concern about the impact on their children with autism.

Many were worried that some kids would not fit at all on the autism spectrum and miss out on cognitive and behavioural therapies.

But Canadian autism expert Katherine Moxness, the executive director of the West Montreal Readaptation Centre, said the new DSM-5 should not change the work of clinicians who follow best practice guidelines when determining where a child or adult fits on the autism spectrum.

What concerns her is how the revised scale of ASD severity will be interpreted and who will be making the proper diagnosis in order to ensure timely therapies for children with autism.

Moxness told CTVNews.ca that new autism definitions and guidelines will actually improve the way cases are evaluated, but waiting for a diagnosis could take months, or years.

In Montreal, for example, it could take two years to get a diagnosis, she said.

Kevin Stoddart, the director of the Redpath Centre in Toronto whose clinical focus has been children, youth and adults with mild autism spectrum disorders, said the new diagnostic manual hasn’t been in effect long enough to measure its impact.

However, “we are all concerned about people with the higher-functioning (ASD),” he told CTVNews.ca.

“We’re worried, but we’ll have to wait and see how it works out.”

Proper diagnosis and treatment is already problematic because Canada doesn’t have a national autism strategy, Stoddart said, and services can be “incredibly fragmented.”

As for Asperger’s disorder, both Moxness and Stoddart said the term won’t be simply dropped from clinical use in Canada overnight because it’s no longer in the DSM.

“Asperger’s has its own identity,” Moxness said. “And the Asperger’s population still requires services.”