National health policy in England may be based on a substantial underestimate of total autism prevalence, according to a new meta-analysis led by researchers from University College London (UCL).
Their study, published in Lancet Regional Health Europe, estimated that there were approximately 750,000 undiagnosed autistic adults in England. This would bring the total of diagnosed and undiagnosed autism cases to around 1.2 million, about twice as many as the figure usually cited by the Government for the entire UK.
The researchers explained: "Autism has long been viewed as a paediatric condition, meaning that many autistic adults missed out on a diagnosis as children, when autism was little known."
In order to estimate the prevalence of undiagnosed adult autism in England, researchers conducted a population-based cohort study of prospectively collected primary care data harvested from IQVIA Medical Research Data and included details of over six million English GP practice patients (5,586,100 individuals registered between 2000 and 2018, and 602,433 individuals registered in 2018).
They compared the prevalence of diagnosed autism in the primary care database with results of a literature search of PubMed up to 17 October 2022 extracting studies showing rates of community case-finding, which the team said should identify the numbers of both diagnosed and undiagnosed autistic individuals in a population.
The comparison of formally diagnosed primary care cases with community prevalence data was then used to estimate underdiagnosis of autism.
Diagnosed Autism Over 100 Times More Common in Children Than Older Adults
As expected, rates of formally-diagnosed autism were much higher in children and young people than in adults, particularly older adults. As of 2018, the diagnosis rate was 2.94% (1 in 34) for those aged 10-14 versus 0.02% (1 in 6000) among those aged 70 and over.
Age-related differences were also evident in incidence figures from 2018, with around 1 in 250 of those aged 5-9 having a newly-recorded autism diagnosis, compared with 1 in 4000 at age 20-49, and 1 in 18,000 among people aged over 50.
The researchers said: "Substantial age-related differences in the proportions of people diagnosed suggest an urgent need to improve access to adult autism diagnostic services."
Their exploratory projections based on the comparative data suggested that, as of 2018, 463,500 people (0.82% of the English population) may have been diagnosed autistic, and between 435,700 and 1,197,300 could be undiagnosed autistic, representing 59-72% of autistic people, and 0.77% to 2.12% of the English population.
Diagnostic Criteria Now 'More Inclusive'
The researchers said that the lower estimate was based on the widely stated figure from epidemiological research in 2011, "before changes to the diagnostic criteria for autism that made them more inclusive", that around 1% of people in England are autistic. Whereas the upper estimate of around a 3% prevalence was based on rates of diagnosed autism in young people (aged 10-19) in the dataset.
They said that this was because "young people are most likely to have had their autism recognised, since providers are now very aware of autism in young people".
Their estimates suggested that between 150,000 and 500,000 people aged 20-49 years and between 250,000 and 600,000 people over the age of 50 may be autistic but undiagnosed.
The midpoint of these figures translates to approximately 750,000 undiagnosed autistic people aged 20 and above, in England. This brings the estimated total autistic population to over 1.2 million – approaching double the figure of 700,000 cited by the government for the whole UK.
Autism 'Under-Recognised in Adults'
Lead researcher, post-doctoral researcher Elizabeth O'Nions PhD, a post-doctoral researcher at UCL , said: "Historically, autism has been considered as a condition of childhood. But recently, awareness has been growing that it is present across the lifespan – in adults as well as young people.
"Nevertheless, autism is still under-recognised in adults. Our estimates suggest that about 180,000 people aged 20-plus had an autism diagnosis as of 2018, meaning that most autistic adults in England were undiagnosed.
She added: "This may partly reflect a lack of awareness and understanding of autism in adults on the part of healthcare professionals. Older adults may also be less likely to self-identify as autistic, meaning that they do not come to the attention of services.
"Meanwhile, providers may be hesitant to raise the issue of autism given the uncertainty around waiting times for a diagnosis and the availability of support or specialist services post-diagnosis."
An Autism Diagnosis can be 'Life Changing'
Dr Judith Brown, head of evidence and research at the National Autistic Society, which worked in partnership with UCL on the study, told Medscape News UK: "An autism diagnosis can be life changing and is vital to getting help and support. Without this, too many autistic people are left struggling without the right support at work and home, and could develop mental illness like anxiety or depression, and end up in crisis or even in hospital.
"But waiting times for diagnostic assessments are a critical, nationwide issue. Without significant, long-term funding for diagnosis services across the country, people will continue to wait many months or even years for a diagnosis."
Dr Brown hoped the Government would address the long waits for diagnostic assessment.
"The Government must urgently invest in rolling out these services, as set out in the national autism strategy for England, and make sure that autistic people and their families get the support they need."
She added that said the study drew "much needed" attention to the issues surrounding diagnosis of older adults
"We want to see more research in this area, as we know it is of great importance to many autistic people."
Autistic People Often Have Mental Health Needs
Dr Conor Davison, autism champion at the Royal College of Psychiatrists, which part-funded the study, told Medscape News UK: "The first step towards helping autistic people is identifying that they have it, which is why it's so important that we are able to properly measure the prevalence of autism.
"Autistic people have much higher rates of mental health problems than the general population and thus very often present to mental health services. It is, therefore, crucial that all mental health professionals, including psychiatrists, have a good understanding of autism in order to identify it early and make appropriate reasonable adjustments to mental health treatment."
Dr Davison also called for more research into the prevalence of autism.
"We urgently need more research into this issue as, without knowing the true prevalence of autism it is difficult to allocate the appropriate amount of resources to specialist autism and mental health services," he said. "This study adds to the evidence that we need more investment to reduce waiting times for autism assessments and ensure autistic people can access the support they need."
The study was funded by Dunhill Medical trust, Economic and Social Research Council, Medical Research Council, National Institute for Health and Care Research, Wellcome, and the Royal College of Psychiatrists (RCP0. EO received a post-doctoral fellowship from the Dunhill Medical Trust, which funded the study. JS was supported by the ESRC and NIHR, MR by the MRC, JEJB by the Wellcome Trust and the RCP, JM by the NIHR Applied Research Collaboration South London, and WM by the MRC and NIHR.