The findings, published in the journal Autism Research, show that despite growing awareness about autism, it is still under-diagnosed, particularly in the black and hispanic people in the US.
"There may be various reasons for the disparity, from communication or cultural barriers between minority parents and physicians to anxiety about the complicated diagnostic process and fear of stigma," said study co-author Walter Zahorodny, Associate Professor at Rutgers University in US.
"Also, many parents whose children are diagnosed later often attribute their first concerns to a behavioral or medical issue rather than a developmental problem," Zahorodny added.
According to the study, researchers analysed the education and medical records of 266,000 children who were 8 years old in 2014, seeking to determine how many of those who showed symptoms of the disorder were not clinically diagnosed or receiving services.
Of the nearly 4,500 children identified, 25 per cent were not diagnosed.
Most were black or Hispanic males with deficits in mental abilities, social skills and activities of daily living who were not considered disabled, the research said.
Screening all toddlers, pre-school and school-age children for autism could help reduce the disparities in diagnosis, according to the researchers.
In addition, experts can overcome communication barriers by using pictures and employing patient navigators to help families understand the diagnosis process, test results and treatment recommendations, the study suggested.
"States can help improve access to care by requiring insurance companies to cover early intervention services when a child is first determined to be at risk rather than waiting for a diagnosis," Zahorodny said.
The research was conducted through the Autism and Developmental Disabilities Monitoring Network, a surveillance programme funded by the US Centres for Disease Control and Prevention that tracks the prevalence of the developmental disorder in 11 states.