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Monday, September 23, 2013

Debunking a Medical Test for Autism

Previous posts have discussed the possibility of  medical tests of children or pregnant mothersAt Forbes, Steven Salzberg  writes of a a purported link between autism and maternal antibodies (see an earlier post):
Perhaps the biggest red flag is that the two lead authors, Daniel Braunschweig and Judy Van de Water already have a patent on the proteins described in their paper, and Van de Water is involved with a company, Pediatric Biosciences (PBI), that is already marketing a test to predict autism based on this study. Van de Water is the Chief Scientific Advisor for the company, which has licensed her patent for this specific test.
 ...Van de Water and her friends at Pediatric Biosciences seem far more interested in making money off the fears of prospective parents. PBI is planning to charge about $800 for their test, which they’ll begin selling next year. Stanford University biostatistician Steven Goodman says that they are “peddling false hope that giving birth to autistic kids can be avoided.”
A news article in Science last week raised many serious questions about Van de Water’s study and its claims. Several scientists quoted in that article questioned the premise of the study: that antibodies from the mother somehow get into the brain of the developing fetus. Yale scientist George Anderson said the data are too preliminary and the statistics too weak to support a clinical test (essentially the same argument I’m making here).
From the Science article:
Financial considerations aside, the key to any medical diagnostic test is its positive predictive value, a number that indicates how often a test, if positive, is right, [Yale's George] Anderson says. In a disorder such as autism, which has a prevalence in the general population of about one in 88 births, even a few false positives very quickly render a test of limited value, he says. Based on his calculations using Van de Water's data and the prevalence of autism, "I get a positive predictive value of 16.5%, which is pretty bad." At that rate, only one out of six positive tests would actually be correct, he explains.
[Stanford's Steven] Goodman says that he came up with the same low positive predictive value after crunching the data provided in Van de Water's paper. "That assumes that you accept these numbers at face value, and I don't," he says. Based on the design of the study and his own analysis of the group's published data, he describes Pediatric Bioscience's claim that a positive result on a test for these antibodies will mean that a woman has a "99 percent" likelihood of having a child with autism as "completely false."
Van de Water and D'Alvise respond that the company's press release was "a bit unclear as written" and that likelihood "was not meant to convey likelihood in the statistical sense, but rather the 99% accuracy with which the study demonstrated specificity of the biomarkers for ASD." They also say that test is not meant to screen the general population but would be for women at higher risk of having autistic offspring, such as those who are older or who already have a child with developmental issues.