While doctors and researchers are aware that children may have diagnoses of both attention-deficit/hyperactivity disorder (ADHD) and autism, a recent study has discovered that an early diagnosis of ADHD may lead to a late diagnosis of Autism Spectrum Disorder (ASD). Despite the co-occurrence of the disorders, researchers have found that an ADHD diagnosis may delay the diagnosis of ASD up to three years. According to the research published by Pediatrics journal, the delay in diagnosis “persists across age and severity of the ASD”—even with severe symptoms of autism present, if ADHD is diagnosed first, any symptoms may be considered a result of the ADHD rather than autism.
One of the main reasons the delay occurs could be due to the similar symptoms the two disorders often exhibit. Clinicians may not consider testing for ASD once an ADHD diagnosis has been given. 
However, the reverse is not true, according to the study, if a child is first diagnosed with autism. The head researcher, Dr. Amir Miodovnik, suggests that this difference may have to do with the way ADHD manifests in young children, writing that these children may “exhibit unique dimensional traits that could bias clinicians toward an ADHD diagnosis.” This could be attributed to symptoms specific to ASD, such as “social communication challenges and repetitive behavior” or not being observed at a younger age, according to Autism Speaks, an autism advocacy organization.
In some ways this research comes as no surprise—both ADHD and ASD are complex conditions and often have overlapping diagnoses. This research, however, does highlight the importance of individualized and rigorous evaluation. Autism Speaks’ head medical researcher, Dr. Paul Wang, explains, “Sometimes it takes evaluation by a team of experts to tease symptoms apart.” Dr. Wang stresses the importance of children receiving the right diagnosis early on and urges parents or those close to the child to voice any concerns. Based on this research, many children who are diagnosed with ADHD do not receive an ASD diagnosis until they are about 6 years of age, when most children on average are diagnosed with autism at around 4 years of age; an earlier diagnosis can make sure that children are receiving the interventions and treatments they need. Dr. Miodovnik recommends that clinicians follow the “American Academy of Pediatrics guidelines for universal screen for ASD at 18 and 24 months of age” to be sure the symptoms of ADHD are not overshadowing those of ASD.
Current research has made diagnosing ASD at a younger age not only possible, but also reliable—one recent study has even used smell as a form of nonverbal evaluation, which helps when diagnosing young children. If your child has been diagnosed with ADHD and you have questions about the diagnosis, or if you would like more information about the co-occurrence of ADHD