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Changes in Prevalence and Age at Diagnosis

Erik T. Parner, PhD; Diana E. Schendel, PhD; Poul Thorsen, PhD

Arch Pediatr Adolesc Med. 2008;162(12):1150-1156.

Objective  To examine the effect of changing age at diagnosis on the diagnosed prevalence of autism among different birth cohorts.

Design  Population-based cohort study.

Setting  Children were identified in the Danish Medical Birth Registry and psychiatric outcomes were obtained via linkage with the Danish National Psychiatric Register.

Participants  All children born in Denmark from January 1, 1994, through December 31, 1999 (N = 407 458).

Main Outcome Measures  The age-specific prevalence, hazard ratio, and relative risk by age.

Results  Statistically significant shifts in age at diagnosis were observed for autism spectrum disorder; children diagnosed before age 9 years in the cohorts born between January 1, 1994, and December 31, 1995, between January 1, 1996, and December 31, 1997, and between January 1, 1998, and December 31, 1999, were on average diagnosed at ages 5.9 (95% confidence interval [CI], 5.8-6.0), 5.8 (95% CI, 5.7-5.9), and 5.3 (95% CI, 5.2-5.4) years, respectively. The relative risk comparing the 1996-1997 birth cohort with the 1994-1995 birth cohort at age 3 years was 1.20 (95% CI, 0.86-1.67), which decreased to 1.10 (95% CI, 1.00-1.20) at age 11 years. Similarly, the relative risk comparing the 1998-1999 birth cohort with the 1994-1995 birth cohort at age 3 years was 1.69 (95% CI, 1.24-2.31), which decreased to 1.23 (95% CI, 1.11-1.37) at age 11 years. Similar results were observed for childhood autism.

Conclusions  Shifts in age at diagnosis inflated the observed prevalence of autism in young children in the more recent cohorts compared with the oldest cohort. This study supports the argument that the apparent increase in autism in recent years is at least in part attributable to decreases in the age at diagnosis over time.

Author Affiliations: Department of Biostatistics (Dr Parner) and North Atlantic Neuro Epidemiology Alliances, Department of Epidemiology (Drs Parner and Thorsen), Institute of Public Health, University of Aarhus, Aarhus, Denmark; and National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention (Dr Schendel), and Department of Epidemiology, Rollins School of Public Health, Emory University (Dr Thorsen), Atlanta, Georgia.

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