Diagnostic stability in individuals with autism spectrum disorder: insights from a longitudinal follow‐up study

BACKGROUND: This longitudinal study of autism symptom trajectories provides unique information that can characterize autism features and diagnostic patterns from childhood to adulthood. METHODS: Participants (n = 155) were part of a longitudinal cohort referred for possible autism where in‐person assessments were completed at ages 2, 3, 5, 9, 19, and 25. Assessors were blinded to previous diagnoses. Based on adult best estimate diagnoses, participants were categorized into one of the four groups: Retained ASD, Lost ASD, Never Had ASD, or Gained ASD Diagnosis. To examine developmental changes in autism symptoms, mixed models indicated the rate of change in ADOS CSS and ADI‐R scores in each diagnostic group. RESULTS: A subset of participants with VIQ> and <70 were assigned a diagnosis in adulthood that differed from diagnoses earlier in development. Across cognitive levels, the majority of novel diagnoses emerged in adulthood. For those with VIQ > 70, improvements in ADOS CSS over time for the Lost Diagnosis group and worsening in CSS in the Gained Diagnosis group were gradual. Individuals with VIQ > 70 who lost a diagnosis even in adulthood could be distinguished on CSS and ADI‐R scores by age 5 from those who retained their ASD diagnosis. Although most participants with VIQ < 70 saw decreases in autistic symptoms as a whole, changes in autism diagnoses were confounded by disentangling profound intellectual disability as a differential diagnosis or co‐occurrence. Only the Never Had Diagnosis group revealed significant changes in ADOS scores over time, with autism symptoms increasing. CONCLUSIONS: Associated with gradual changes in core features of autism beginning in childhood, diagnoses of autism can shift across development.