A wide spectrum of studies conducted in numerous locales suggest that the measured prevalence of autism, a condition characterized by impairments in social interaction and communication, has increased markedly over the past 40 years.1
Consistent with previous studies, data from the California Department of Developmental Services (DDS) show that between 1987 and 2003 the number of autism cases handled by the California DDS increased by 634%.2
The general consensus is that these increases are striking. Equally striking is the absence of consensus regarding the degree to which changes in diagnostic standards, practice and procedures, and/or diagnostic substitution have contributed to increased prevalence rates.
Some scholars have suggested that diagnostic substitution plays a significant role in the increasing prevalence of autism.3–8
Diagnostic substitution occurs when an individual, net of changes to diagnostic standards, practices and procedures, or individual condition, is diagnosed with one condition at one time and subsequently with another condition at some further point in time. One reason why diagnostic substitution seems plausible is that autism is difficult to diagnose since there are no known biological markers and the symptoms are hard to assess, especially among persons with cognitive impairments. Evidence in support of the diagnostic substitution hypothesis arises from recent studies which have shown that increased autism rates are accompanied by concurrent declines in the prevalence of mental retardation (MR) and other developmental disabilities.3,5,7
At the same time, other studies have found no evidence of diagnostic substitution.9,10
Independent of diagnostic substitution, some scholars have argued that changes in diagnostic practices lie behind the increased prevalence of autism. Of course, these scholars also note that changing diagnostic practices and procedures may both accompany and be implicated in a process of diagnostic substitution. Since Kanner first described autism in 1943, diagnostic standards, practices and procedures have changed considerably.8,11,12
Some local studies have shown that changing diagnostic practices has a demonstrable effect on autism incidence rates, for example, in Olmstead County, Minnesota.13
A study applying current autism diagnostic criteria to adults who had a history of developmental language disorder, found that 32% (12/38) of adults qualified for an autism spectrum diagnosis under the new standards.5
Similarly, a recent study examining population incidence rates in California found that changes in diagnostic criteria may account for as much as one-third of the increased prevalence in the state.14
Hence, there is some evidence that both changes in diagnostic standards and diagnostic substitution are driving part of the observed increase in prevalence. What is less well understood is how these two processes are related and the effect they may have on the prevalence of autism.
Much of the controversy over diagnostic substitution and diagnostic change has focused on data from the California DDS. Analyses and subsequent re-analyses differ in the extent to which they contend that the increase in measured prevalence arises from diagnostic substitution and/or changes in diagnostic standards. Croen et al
initially suggested that much of the increase in prevalence could be attributed to diagnostic substitution. However, Blaxill et al
identified several concerns in the paper by Croen et al
., including problematic analysis of trend information and evidence of ascertainment bias in younger cohorts. Rethinking these issues from the original paper, Blaxill et al
. concluded that there was little evidence to support the idea that diagnostic substitution played a significant role in increasing autism prevalence.16
Croen et al
. agreed, concluding, ‘[d]iagnostic substitution does not appear to account for the increased trend in autism prevalence’.17
Likewise, a separate study of special education records concluded that diagnostic substitution is not occurring in California, though it does appear to be happening in the majority of other states.7
In sum, although several studies have found that diagnostic substitution appears to be occurring in several locations, the evidence suggests that it is not occurring in California.
Comparing aggregated prevalence rates for autism and MR or other disabilities, rather than looking at changes in individual cases, has hampered previous studies. In fact, with only one exception,5
studies have looked at aggregated prevalence rates to identify diagnostic substitution, despite the fact that diagnostic substitution happens at the level of the individual. Comparing aggregated prevalence rates to identify diagnostic substitution can lead to erroneous conclusions. For instance, in contexts where the overall MR caseload was increasing and diagnostic substitution was occurring, a comparison of MR and autism caseloads would lead to the faulty conclusion that diagnostic substitution was not taking place. Unfortunately, one can learn little about the dynamics of diagnostic change by focusing solely on the macro level.
In this article, we consider the effect of diagnostic change at both the individual and macro levels. At the individual level, one form of diagnostic change is diagnostic substitution in which there is a switch in diagnosis (X → Y). A second form of diagnostic change at the individual level is what we identify as diagnostic accretion. Diagnostic accretion occurs when an individual initially diagnosed with one disorder subsequently acquires a second diagnosis, but retains the first diagnosis as a co-morbidity (X → X + Y). Diagnostic accretion, should it be occurring, would impact the autism caseload but would have no discernable affect on the MR population, thus making the process difficult to identify in the aggregate. At the macro level, diagnostic change refers to changes in diagnostic standards and practices that propel either substitution or accretion. Here too, we note that the aggregation of cases without disaggregating micro and macro level processes can obscure the effect that diagnostic changes may have on autism caseloads.
We seek to estimate the impact of diagnostic change on the prevalence of autism. Recall that diagnostic change captures diagnostic substitution and accretion, which may reflect changes in diagnostic practices. Changing diagnostic practices may include changes in diagnostic standards, such as a change to the Diagnostic and Statistical Manual of Mental Disorders (DSM), as well as the procedures or guidelines used when making a diagnosis. In many administrative systems, such as special education classifications, clients are assigned a single diagnosis. In contexts such as these, clinicians are allowed to check one box, and we would observe pure diagnostic substitution if persons who would have previously been diagnosed with one disorder are now diagnosed with autism. However, in the California DDS systems, clients may maintain multiple diagnoses. Thus, to observe diagnostic accretion, we can look for clients who enter the system with a single diagnosis and subsequently acquire a second diagnosis.
We are particularly interested in when these clients experience diagnostic substitution or accretion, as one of the central research questions that confronts us is whether or not changes in diagnostic practices impact individuals’ diagnostic state. For instance, if an individual had a sole diagnosis of MR under the DSM-III guidelines, but was later additionally diagnosed with autism under DSM-IV criteria, resulting in MR–autism co-morbidity, we would observe a diagnostic accretion as the result of a change in diagnostic standards. Similarly, if an individual was diagnosed with MR under one diagnostic regime and under a later diagnostic regime had a sole diagnosis of autism, this would be a diagnostic change (as a diagnostic substitution) catalysed by changing diagnostic standards or practices.
Relevant for this study are changes in diagnostic practices in California since 1987, that is, since publication of the DSM-III-R. Diagnostic standards were changed in 1994 (DSM-IV), 1998, 2000 (DSM-IV-TR) and 2001–02. The 2000 DSM-IV-TR changed the criteria for pervasive developmental disorder—not otherwise specified (PDD-NOS), which could influence autism diagnoses, though the revision did not specifically address the diagnostic criteria for autism. The 1998, 2001 and 2002 revisions were changes in practices specific to California. Beginning in 1998 and continuing through 2002, a series of changes in diagnostic practices were implemented by the DDS. In 1998, the first ‘Best Practices’ guide was published by the Department of Developmental Services and the California Department of Education. In 2001, increasing attention was directed to the diagnostic component of best practices as dictated by Assembly Bill 430 (effective August 2001), which required the DDS to develop evaluation and diagnostic procedures for the diagnosis of autistic disorder, as well as to create a training program for the utilization of the new diagnostic guidelines. The bill was passed on 10 August 2001 and required the DDS to develop evaluation and diagnostic procedures by 1 April 2002 and implement the training program by 1 July 2002. A rough proxy for the complexity of the diagnostic practice is the manual's length—it is 180 pages long.
Throughout this process the diagnostic practices and procedures underlying the autism spectrum as a whole have changed and the boundaries of autism spectrum disorders have blurred. In this article, we examine the increasing prevalence of autistic disorder (DSM 299.00). When references are made to autism spectrum disorders, including Asperger's Disorder and PDD-NOS, we will be explicit. In all other instances, autism refers to autistic disorder.