In medicine, a differential diagnosis is a technique physicians sometimes use to identify the cause of a patient’s symptoms. As its name indicates, the process is intended to result in a medical diagnosis.

The term “differential diagnosis” has often been used in toxic tort litigation to mean something completely different. A plaintiff who files a toxic tort case should already have a medical diagnosis.  Indeed, it should be the diagnosis which leads the plaintiff to file the lawsuit in the first place. The toxic tort “differential diagnosis” is not a diagnosis at all, but rather a method whereby a plaintiff’s expert purports to “rule in” various potential causes for the plaintiff’s illness and then to “rule out” alternative causes until only one cause remains. It should come as no surprise to defense attorneys that, when a plaintiff’s expert uses the technique, it inevitably points to the defendant.

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