The 5th Circuit Knows Why Differential Diagnosis Can't Be Used To Establish "General Causation"
In Johnson v. Arkema the plaintiff alleged that he had developed chemical pneumonitis as a result of exposure to hydrochloric acid (HCl) and an organotin compound (MBTC) during the course of his work in a factory manufacturing glass bottles. A fume hood made by Arkema, purchased by his employer and used to draw off HCl and organotin fumes after the substances are applied to the bottles as part of the manufacturing process, allegedly failed to work properly. The result, Johnson claimed, was that he was exposed to the fumes and subsequently suffered both acute and chronic lung injuries. However, following the magistrate's recommendation, the trial court granted summary judgment in favor of Arkema; determining that plaintiff's experts had failed to reliably opine as to causation.
The causal reasoning by plaintiff's expert here was particularly bad. The fact that some lung irritants cause fibrosis at high levels hardly proves that all lung irritants cause fibrosis at all levels of exposure. Similarly, a study suggesting that extraordinarily high levels of exposure to the substance increased the risk of pulmonary fibrosis in baboons is hardly evidence that a tiny fraction of that exposure over two days caused a particular case of pulmonary fibrosis in a human. What makes the opinion blogworthy though is the takedown of plaintiff's attempt to use differential diagnosis to "rule in" a substance as a cause of pulmonary fibrosis when no scientific study has ever shown the substance to produce the malady in humans.
Holding that "an expert may not rely on a differential diagnosis to circumvent the requirement of general causation" the court concluded"
Thus, before courts can admit an expert's differential diagnosis, which, by its nature, only addresses the issue of specific causation, the expert must first demonstrate that the chemical at issue is actually capable of harming individuals in the general population, thereby satisfying the general causation standard.
That general causation can't reliably be established merely by having a doctor pick the one putative cause that seems most plausible out of a group of untested or unverified putative causes is pretty obvious but some courts continue to think otherwise. Take for instance the recent ruling in the consumer "popcorn lung" case of Watson v. Dillon.
Relying on Hollander v. Sandoz, and thereby on Turner v. Iowa Fire Equipment, Co., the trial court essentially concluded that doctors who treat patients possess some faculty whereby they are able to discern that a thing is capable of causing illness simply by process of elimination. The idea comes from Turner and finds its way into a variety of cases supporting plaintiffs' novel causation claims despite the fact that Turner resulted in the exclusion of junk science - but only because plaintiff's expert did not "scientifically eliminate" (whatever that means) the other possible causes of her injury. Here's the much quoted reasoning:
The first several victims of a new toxic tort should not be barred from having their day in court simply because the medical literature, which will eventually show the connection between the victims' condition and the toxic substance, has not yet been completed. If a properly qualified medical expert performs a reliable differential diagnosis through which, to a reasonable degree of medical certainty, all other possible causes of the victims' condition can be eliminated, leaving only the toxic substance as the cause, a causation opinion based on that differential diagnosis should be admitted.
But on what basis should such an opinion be admitted? Is there anything beyond the assumption that if something is "toxic" at some level then it's probably capable of causing whatever afflictions mankind suffers unless science has ruled it out? Nope. It's thus just another example of argumentum ad ignorantiam . And where did the idea that toxins ought to be presumptively "ruled in" come from? Once again, it's the idea that we are living in "The Age of Degenerative and Man-Made Diseases" - that we're reaping the bitter harvest of modernity. Too bad they didn't have the PhyloChip back then when chemophobia was hatched - a time when it was believed that the microbial world was not only understood but conquered. A lot of time, money and effort might not have been wasted. And who knows how many lives might have been saved?
