Finding out if a child is on the autism spectrum could be as simple as taking a blood test.
Researchers in the US have found a link between autism spectrum disorder (ASD) and the lack of an antibody called immunoglobulin G type 1, or IgG1.
Combined with other testing, such as hormone level measurements, the researchers say ASD could be identified in children before they start to show obvious symptoms.
"Numerous investigators have long sought a biomarker for ASD," says Dwight German, study senior author and Professor of Psychiatry at UT Southwestern Medical School in Texas.
"The blood biomarker reported here, along with others we are testing, can represent a useful test with over 80 percent accuracy in identifying ASD."
According to the study, published in journal Scientific Reports, it's presently difficult to diagnose ASD in children before the age of four, when their difficulties in socialising and communicating become more apparent.
Early intervention is key to getting these children the help they need, says Dr German.
The lack of IgG1 correlated closely to the severity of ASD, at least in boys; there was only a "small ratio" of girls included in the study, and the correlation wasn't as strong as it was in boys, so Dr German says more research is needed.
Researchers say little is known about what causes ASD, but there is evidence to suggest it has a genetic basis.
There is no evidence, at all, behind the popular theory it's caused by vaccines.