Assessment of Autism Symptoms From 6 to 18 Months of Age Using the Autism Observation Scale for Infants in a Prospective High‐Risk Cohort

The objectives were to characterize behavioral signs of autism spectrum disorder (ASD) in younger siblings of diagnosed children (high‐risk; HR) and examine classification features of the Autism Observation Scale for Infants (AOSI). Participants (501 HR and 180 low‐risk [LR]) were assessed between 6 and 18 months using the AOSI and at age 3 for ASD diagnoses. Total AOSI scores differentiated HR infants later diagnosed with ASD starting at 12 months. ROC analyses identified 12‐ and 18‐month cutoff scores associated with 0.52 sensitivity and 0.74 specificity and 0.73 sensitivity and 0.65 specificity, respectively. Although classification accuracy does not support use as a standalone screen, the AOSI identifies features associated with ASD starting at 6 months and differentiates HR infants with ASD by 12 months.