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More people were diagnosed with autism spectrum disorders.

Many have noticed that autism spectrum disorders (ASD) seem to be surfacing more frequently, but is it because we are getting better at recognizing them? A Danish study analyzed the incidence rates of autism over the last two decades. The researchers expected to see an increase in diagnoses, but they did not know which groups would have the largest increases. According to the study, diagnosis rates increased in all populations, but diagnoses among girls and adults demonstrated a particularly rapid rise.

The researchers utilized records from the Danish Psychiatric Central Research Registry to determine the incidence of ASD in the population. They tallied the number of new diagnoses made from 1995 to 2010. To ensure standardized diagnostic criteria, they used definitions based on the International Classification of Diseases, 10th edition, the ICD-10 (the DSM-V’s international counterpart). Although forms of autism like Asperger’s syndrome pervasive developmental disorder-not otherwise specified (PDD-NOS) are no longer part of the DSM, they are still valid diagnoses according to the ICD-10.

Nearly 15,000 people in Denmark received an ASD diagnosis during the study interval, increasing the overall incidence of autism from 9 to 38.6 per 100,000 people. Male diagnosis rates quadrupled (13.2 to 58 per 100,000) and female diagnosis rates increased sevenfold (2.6 to 18.6 per 100,000). The average diagnosis age was nine for boys and 11 for girls.

The biggest increases came from some unexpected groups. The diagnosis of girls with Asperger’s or PDD-NOS increased the most. Girls saw an 11-fold jump in PDD-NOS diagnoses (0.8 to 9.3 per 100,000) and a fivefold increase in Asperger’s (0.6 to 5.8 per 100,000). The fastest-growing age group for new ASD diagnoses was the 14 to 20 cohort; adults aged 21 to 65 represent nine percent of new autism diagnoses.

The findings suggest that the increasing rate of autism prevalence is, at least partially, due to recognizing autism in people who were not diagnosed as children. This may be thanks to better and more frequent screenings, or new clinicians entering the field with a stronger understanding of autism. The research also contradicts the idea that boys are more affected by autism than girls. The diagnosis of girls with PDD-NOS and Asperger’s may indicate that autism is expressed differently in females.

This research is published in the Journal of Autism and Developmental Disorders.

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