Diagnostic change and the increased prevalence of autism

Abstract

Background: Increased autism prevalence rates have generated considerable concern. However, the contribution of changes in diagnostic practices to increased prevalence rates has not been thoroughly examined. Debates over the role of diagnostic substitution also continue. California has been an important test case in these controversies. The objective of this study was to determine the extent to which the increased prevalence of autism in California has been driven by changes in diagnostic practices, diagnostic substitution and diagnostic accretion.

Methods: Retrospective case record examination of 7003 patients born before 1987 with autism who were enrolled with the California Department of Developmental Services between 1992 and 2005 was carried out. Of principal interest were 631 patients with a sole diagnosis of mental retardation (MR) who subsequently acquired a diagnosis of autism. The outcome of interest was the probability of acquiring a diagnosis of autism as a result of changes in diagnostic practices was calculated. The probability of diagnostic change is then used to model the proportion of the autism caseload arising from changing diagnostic practices.

Results: The odds of a patient acquiring an autism diagnosis were elevated in periods in which the practices for diagnosing autism changed. The odds of change in years in which diagnostic practices changed were 1.68 [95% confidence interval (CI) 1.11-2.54], 1.55 (95% CI 1.03-2.34), 1.58 (95% CI 1.05-2.39), 1.82 (95% CI 1.23-2.7) and 1.61 (95% CI 1.09-2.39). Using the probability of change between 1992 and 2005 to generalize to the population with autism, it is estimated that 26.4% (95% CI 16.25-36.48) of the increased autism caseload in California is uniquely associated with diagnostic change through a single pathway--individuals previously diagnosed with MR.

Conclusion: Changes in practices for diagnosing autism have had a substantial effect on autism caseloads, accounting for one-quarter of the observed increase in prevalence in California between 1992 and 2005.

Figure 1

Number of diagnostic changes (accretions or substitutions) by year. Vertical arrows denote years in which diagnostic practices change

Figure 2

The cumulative adjusted probability of an average individual experiencing a change in diagnosis in a given year. The baseline probability, which takes into account changes in the underlying population distribution, is represented by the line marked with a triangle, and ranges from 0.003 to 0.004. The adjusted probability of observing a change allowing for observed revisions of diagnostic practices is represented by the line marked with an asterisk. In 2005, the cumulative probability of change allowing for new practices was 0.13

Figure 3

The changing diagnostic world in California between 1992 and 2005. The number of cases of autism and MR both expand, though the proportion of autism cases is increasing at a much greater rate. As this is happening, the rate of co-morbidity within the autism population is decreasing, though the number of co-morbid cases is increasing, as is co-morbidity within the population with MR. Represented in the non-shaded portion of the MR circles is MR*, the number of cases of MR without co-morbidity

Figure 4

Autism caseload increase in California between 1992 and 2005. Shaded in grey is the observed increase in DDS autism cases. In black is the proportion of that increase that is due to diagnostic change. 95% CIs are denoted by white bars