Patricia M. Dietz, Charles E. Rose, Dedria McArthur & Matthew Maenner have an article at The Journal of Autism and Developmental Disorders titled "National and State Estimates of Adults with Autism Spectrum Disorder." The abstract:
U.S. national and state population-based estimates of adults living with autism spectrum disorder (ASD) are nonexistent due to the lack of existing surveillance systems funded to address this need. Therefore, we estimated national and state prevalence of adults 18–84 years living with ASD using simulation in conjunction with Bayesian hierarchal models. In 2017, we estimated that approximately 2.21% (95% simulation interval (SI) 1.95%, 2.45%) or 5,437,988 U.S. adults aged 18 and older have ASD, with state prevalence ranging from 1.97% (95% SI 1.55%, 2.45%) in Louisiana to 2.42% (95% SI 1.93%, 2.99%) in Massachusetts. Prevalence and case estimates of adults living with ASD (diagnosed and undiagnosed) can help states estimate the need for diagnosing and providing services to those unidentified.From the article:
To date, an empirical study of adult ASD prevalence in the U.S. has not been accomplished, perhaps because any single approach to ascertain adult ASD has challenges. There are no psychometrically validated tests of ASD for adults, which leads to uncertainty for studies using tests designed for children, such as the Autism Diagnostic Observation Schedule. In addition, mixed methods are likely needed in order to reach populations living independently and in group settings. A subset of persons might only be identified through the review of service records of those being served in group settings. Individuals with ASD who live independently may be disinclined to participate in a survey if recruited via phone or in person. Adults with ASD may be more difficult to recruit because they may not be enrolled in services or may not receive services in a wide variety of settings (e.g., schools, health care providers, community based entities) resulting in challenges to comprehensive recruitment efforts. Once a validated tool to identify adults with ASD is created, a study could incorporate information from public school classifications or publicly-funded programs that serve individuals with ASD and population-based telephone or community surveys of adults with adjustments to address greater non-response among adults with ASD.
Overall, we estimated that 1 in 45 adults (95% SI, 41, 51), ages 18–84 years, are living with ASD. While these numbers are estimates, they do provide a place for states to think about available services for adults with ASD. We used the most current data available for all states to estimate the ASD prevalence among adults. This analysis may motivate some states to explore state-based data sources that may be more informative than data available for all states, and refine the estimates based on their existing local data.