Diagnostic Substitution

I am of the opinion that an ongoing increase in the real prevalence of autism is not a foregone conclusion. I sometimes chafe at watching the general mass media, which seems to operate with the baseline assumption that the easily-demonstrated and indisputable increase in the administrative prevalence (i.e. reported numbers) of autism automatically signifies that more autistic people exist now than existed previously. I feel that the jury is still out on whether a real increase has occurred.
Anyone who has spent any time looking at this issue will have a pretty good idea of some of the reasons to believe that the current upward trend in administrative prevalence does not actually mean that more autistics are being born (or "damaged" into autism) nowadays. Among them are several changes in the diagnostic criteria, the adoption of the Spectrum Disorder assignation which includes PDD and Asperger's Syndrome, changes in reporting requirements at various service providers, an increased awareness among diagnosticians and a possible improvement in diagnostic tools, and a reduction in the social and educational stigma associated with Autism. Another chief reason is diagnostic substitution.
Diagnostic substitution describes an occurence in which a person either receives a change in diagnosis or is diagnosed with, in this case, autism instead of a different diagnosis that may have been appropriate under previous diagnostic criteria. It has been demonstrated numerous times that in CDDS data, the increase in administrative prevalence for autism very closely mirrors the simultaneous decrease in administrative prevalence for Mental Retardation. It has also been closely examined that Childhood Schizophrenia is a diagnosis that has, for many children, been replaced by an autism diagnosis as the diagnostic criteria for autism changed and developed over the years.
Setting aside speculation, it is nice to see some science being done on this question. Establishing accurate and consistent real prevalence figures and trends would be tremendously beneficial in understanding many aspects of autism, including clues to causation as well as planning for the future needs of autistic people in a whole-society sense.
Today I saw the abstract for this study that has been published in Developmental Medicine and Child Neurology . An excerpt form the abstract states:

"Rates of diagnosis of autism have risen since 1980, raising the question of whether some children who previously had other diagnoses are now being diagnosed with autism. We applied contemporary diagnostic criteria for autism to adults with a history of developmental language disorder, to discover whether diagnostic substitution has taken place. ... Some children who would nowadays be diagnosed unambiguously with autistic disorder had been diagnosed with developmental language disorder in the past. This finding has implications for our understanding of the epidemiology of autism."

That's pretty straightforward. This study follows another published in the Journal of Autism and Developmental Disorders (2007 Nov). From the abstract:

"We examined trends in assignment of special education codes to British Columbia (BC) school children who had an autism code in at least 1 year between 1996 and 2004, inclusive. The proportion of children with an autism code increased from 12.3/10,000 in 1996 to 43.1/10,000 in 2004; 51.9% of this increase was attributable to children switching from another special education classification to autism (16.0/10,000). Taking into account the reverse situation (children with an autism code switching to another special education category (5.9/10.000)), diagnostic substitution accounted for at least one-third of the increase in autism prevalence over the study period."

So here we have two differently structured studies that both indicate that diagnostic substitution plays a role in the increase in autism administrative caseload prevalence. These two follow another, perhaps more widely known study by Shattuck, published in Pediatrics (Apr 2006). This is the study that resulted in harrassment and character assassination of Paul Shattuck by the folks over at NAA, a major "mercury causes autism" advocacy group. From this study's abstract:

"RESULTS: The average administrative prevalence of autism among children increased from 0.6 to 3.1 per 1000 from 1994 to 2003 ... During the same period, the prevalence of mental retardation and learning disabilities declined by 2.8 and 8.3 per 1000, respectively. Higher autism prevalence was significantly associated with corresponding declines in the prevalence of mental retardation and learning disabilities. The declining prevalence of mental retardation and learning disabilities from 1994 to 2003 represented a significant downward deflection in their preexisting trajectories of prevalence from 1984 to 1993. California was one of a handful of states that did not clearly follow this pattern. CONCLUSIONS: Prevalence findings from special education data do not support the claim of an autism epidemic because the administrative prevalence figures for most states are well below epidemiological estimates. The growing administrative prevalence of autism from 1994 to 2003 was associated with corresponding declines in the usage of other diagnostic categories."

I think that in today's media climate, where fund raising a la Autism Speaks, spotlight seeking a la Jenny McCarthy and David Kirby, and controversy seeking a la every major news outlet are combining to paint a picture of a devastating autism epidemic, it is important to keep in mind that the path to real answer lies in the careful application of science.
And it is even more important to understand and recognize the impact of our focus on various issues on those who sometimes seem to have the quietest voice in a community full of big voices - autistics.