Sep 292012
Reports of autism cases per 1,000 children gre...

Reports of autism cases per 1,000 children grew dramatically in the US from 1996 to 2007. It is unknown how much, if any, growth came from changes in autism’s prevalence. (Photo credit: Wikipedia)

Many people are very concerned with the apparent “explosion” of autism rates. Back in the 80s and 90s, autism prevalence was very low at about 4/10,000. More recent estimates from the CDC currently stand at 1/88, a significant change. As the pattern-seeking animals that we are, many people have made a connection between the apparent rise in autism rates and an expansion of the childhood vaccination schedule, drawing an immature, and scientifically unsupported, conclusion that vaccines must have cause the increase in autism rates.

Alternative explanations have been put forth, the most plausible one being the “casting of a wider net with smaller holes” hypothesis. It is well known that the diagnostic criteria have changed, increasing the number of children that now fall under the autism spectrum disorder. Furthermore, increased scrutiny about autism, and increased awareness can also contribute to more diagnoses, without necessarily meaning that the prevalence of the disease has gone up. In other words, it is entirely plausible that the number of children that suffer from the disorder has not gone up, but we’re looking for it much more and we are using an expanded set of diagnostic criteria, leading to more diagnoses of autism.

Recently a study was published that lends support to the “wider net with smaller holes” hypothesis. Entitled “Autism Spectrum Disorder Reclassified: A Second Look at the 1980s Utah/UCLA Autism Epidemiologic Study” the study abstract says the follwoing (emphasis added):

The purpose of the present study was to re-examine diagnostic data from a state-wide autism prevalence study (n = 489) conducted in the 1980s to investigate the impact of broader diagnostic criteria on autism spectrum disorder (ASD) case status. Sixty-four (59 %) of the 108 originally “Diagnosed Not Autistic” met the current ASD case definition. The average IQ estimate in the newly identified group (IQ = 35.58; SD = 23.01) was significantly lower than in the original group (IQ = 56.19 SD = 21.21; t = 5.75; p < .0001). Today’s diagnostic criteria applied to participants ascertained in the 1980s identified more cases of autism with intellectual disability. The current analysis puts this historic work into context and highlights differences in ascertainment between epidemiological studies performed decades ago and those of today.

Let us think about this for a second; 59% of those who did not meet the previous criteria for autism, meet them under the current guidelines.  Now, the original study in 1990 tried to specifically recruit children with autism, so it is not too surprising that the percentage that was “missed” under the old standards, but would be classified as autistic under the current, broader standards, was so large. Logically, we can only expect that the percentage of such “missed” diagnoses would be much lower in the general population. So let us be very generous and for argument’s sake say that it is only 1% instead of 59%. So if you took a random sample of 10,000 children in 1990 and applied the old standard, and you only came up with 4 diagnoses of autism, you should have had another 100 (1% of 10,000) which you missed.

The same group of 10,000 children would have yielded 4 diagnoses in 1990 vs. 104 diagnoses today, solely due to a different set of diagnostic criteria being used in the two instances. Whatever the real prevalence rate for the group, let’s call it X is, it would not change simply because we are using different diagnostic criteria. Only our estimate of X has changed.

So the important observation is this: If we applied today’s standards back in 1990 we’d get a prevalence rate of 0.0104 (104/10,000). The current CDC estimate today, is 0.0114 (1/88).

0.0104 is practically equal to 0.0114

So, have I just proven that the increase in autism diagnoses is due solely to the expansion of the diagnostic criteria? Certainly not; this is only one study we are talking about here; it needs to be replicated to see if the numbers hold. Further, I took a wild guess in picking 1% as the overall “miss” rate between the old and the current standards. For all I know it could be 5% or 0.1%.

But, this study does provide another piece of evidence against the hypothesis that vaccines are causing the increase in autism diagnoses. It shows that for this sample of children in Utah, the “miss” rate of the old standards  was quite large; it shows that the “wider net with smaller holes” hypothesis has a basis in reality and is supported by the data. And, it is yet another study that fails to support the anti-vaxxers favorite piece of propaganda.

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  One Response to “Study provides evidence for effect of expanded diagnostic criteria on rates of autism”

  1. [...] Study provides evidence for effect of expanded diagnostic criteria on rates of autism, L. Shaka, Sep… [...]

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