Diagnostic Stability Over Two Years Among Younger Siblings of Children with Autism Spectrum Disorders

Background: Previous studies indicate that ASD diagnoses are highly stable over time, with Autistic Disorder being more stable than PDD-NOS. Diagnostic stability among younger siblings of children with ASD has received little attention. Some researchers contend that ASDs manifest themselves differently in multiplex families than in simplex families, suggesting that differences in diagnostic stability between multiplex and simplex families are possible. Objectives: To investigate diagnostic stability among younger siblings of children with ASD from age two to age four. Methods: Thirty-two younger siblings of children with ASD (“siblings”) and 136 children from simplex families (“singletons”) were evaluated twice as part of an early detection of ASD study. Time 1 mean age was 23 months for siblings, 27 months for singletons; Time 2 mean age was 49 months for siblings, 55 months for singletons. Both evaluations included a battery of diagnostic, cognitive, and adaptive tests. Diagnoses were assigned based on the DSM-IV symptom checklist, completed using information from testing and clinical judgment. All children received a diagnosis of Autistic Disorder, PDD-NOS, or non-ASD. Results: Chi-square analysis indicated that diagnostic stability was significant in both the sibling and singleton groups. The Positive Predictive Power (PPP) of an ASD diagnosis at time 1 for an ASD diagnosis at time 2 was .800 in siblings and .798 in singletons, indicating that approximately 80% of children from simplex and multiplex families retained an ASD diagnosis from age two to four. Consistent with other studies, a diagnosis of PDD-NOS was less stable in both groups than a diagnosis of Autistic Disorder. Conclusions: Preliminary results from our small cohort of siblings indicate that ASD diagnoses are similarly stable in siblings as in singletons over two years, and that PDD-NOS is a less stable diagnosis than Autistic Disorder.