UNIVERSITY PARK, Pa., July 23 (UPI) -- The three-fold increase in autism diagnoses in the United States is in large part due to reclassification of people with other disorders, not an increase in people with autism, according to a new study.
Researchers said that some of the reclassification can be attributed to overlap between other neurobiological conditions and autism, as well as a broadening of the diagnostic criteria for autism.
According to the Centers for Disease Control and Prevention, the prevalence of autism has progressed from 1 in 5,000 in 1975, to 1 in 150 in 2002, and 1 in 68 in 2012.
"For quite some time, researchers have been struggling to sort disorders into categories based on observable clinical features, but it gets complicated with autism because every individual can show a different combination of features," Santhosh Girirajan, an assistant professor of biochemistry and molecular biology and of anthropology at Penn State University, said in a press release. "The tricky part is how to deal with individuals who have multiple diagnoses, because the set of features that define autism is commonly found in individuals with other cognitive or neurological deficits."
Researchers used data from the Individuals with Disabilities Education Act, or IDEA, tracking an average of 6.2 million children in special education classes between 2000 and 2010, and found a 331 percent increase in the prevalence of autism during the decade.
Roughly 65 percent of the increase between 2000 and 2010, researchers said, can be attributed to reassignment of children from intellectual disability to autism. For 9-year-olds, about 59 percent of the rise in autism is due to reassignment, and by age 15 reclassifying children can be blamed for 97 percent of the increase.
"Because features of neurodevelopmental disorders co-occur at such a high rate, and there is so much individual variation in autism, diagnosis is greatly complicated, which affects the perceived prevalence of autism and related disorders," said Girirajan. "Every patient is different and must be treated as such. Standardized diagnostic measures incorporating detailed genetic analysis and periodic follow-up should be taken into account in future studies of autism prevalence."
The study is published in the American Journal of Medical Genetics.