It may be possible to diagnose autism by giving children a sniff test, a new study suggests.

Most people instinctively take a big whiff when they encounter a pleasant smell and limit their breathing when they encounter a foul smell.

Children with autism spectrum disorder do not make this natural adjustment, said Liron Rozenkrantz, a neuroscientist at the Weizmann Institute of Science in Israel and one of the researchers involved with the study.

She and her colleagues reported their findings in the journal Current Biology.

They presented 18 children who had an autism diagnosis and 18 typically developing children with pleasant and unpleasant odors and measured their sniff responses. The pleasant smells were rose and soap; the unpleasant smells were sour milk and rotten fish.

Typically developing children adjusted their sniffing almost immediately – within about 305 milliseconds. Children with autism did not respond as rapidly. As they were exposed to the smells, the children were watching a cartoon or playing a video game.

“It’s a semi-automated response,” Ms. Rozenkrantz said. “It does not require the subject’s attention.”

Using the sniff test alone, the researchers, who had not been told which children had autism, were able to correctly identify those with autism 81 percent of the time.

They also found that the further removed an autistic child’s sniff response was from the average for typically developing children, the more severe the child’s social impairments were.

“We hope that it can be used as a diagnostic marker to diagnose autism at a very young age,” Ms. Rozenkrantz said. “This is a nonverbal measure, and it only requires breathing.”

First, though, Ms. Rozenkrantz said more long-term follow-up studies need to be done with young children.