Academic journal article Defense Counsel Journal

Differential Diagnosis and Daubert: Preventing the Misuse of Differential Etiology to Prove Causation in Toxic Tort Cases

Academic journal article Defense Counsel Journal

Differential Diagnosis and Daubert: Preventing the Misuse of Differential Etiology to Prove Causation in Toxic Tort Cases

Article excerpt

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.

The more accurate name for the technique as used in litigation is "differential etiology." Yet, courts persist in calling it a differential diagnosis and sometimes confuse the medical standard for diagnosing an illness with the legal standard for determining cause and effect. When that happens, a plaintiff's expert may be allowed to introduce causation opinions that are based on unscientific speculation.

The case law on differential diagnosis is inconsistent and sometimes contradictory. This article is intended as a guide for defense lawyers who are facing claims in which the plaintiff's expert purports to base his or her causation opinion on a differential etiology. It will explore the method as it has been employed in state and federal courts, and provide some practice tips for defending against such opinions. The goal of this article is to place defense counsel in the best position to exclude differential etiology opinions and, if the opinions survive the defense challenge, to cross examine the expert.

I. Differential Diagnosis in Perspective: The Daubert/Frye Framework and the Plaintiff's Burden

The cases in which courts have admitted speculative differential etiology opinions generally appear to be ones in which the courts have ignored the Daubert or Frye standard and accepted the expert's characterization that his or her causation opinion is based on clinical experience or professional judgment. To be in the best position to keep such testimony out of evidence, the defense lawyer must plan the expert's deposition in such a way that the expert either agrees with the generally-accepted legal standard for proving causation or admits that he or she did not follow that standard. A Daubert or Frye motion should follow.

A. Daubert: Science, not speculation

The purpose of Federal Rule of Evidence 702 is to ensure that any and all scientific testimony or evidence admitted is not only relevant, but reliable. (1) The Supreme Court has explained that an expert must employ in the courtroom the same level of intellectual rigor that characterizes the practice of an expert in the relevant field. (2) The "knowledge" requirement of Rule 702 requires "more than subjective belief or unsupported speculation." (3)

When deciding a Daubert challenge, the judge's task is to separate real science from speculation masquerading as science. Daubert cautioned, however, that the trial court's focus "must be solely on principles and methodology, not on the conclusions that they generate." (4) Since Daubert, appellate courts have sometimes reversed trial courts because the trial courts focused more on the expert's conclusion (with which it disagreed) rather than on the expert's methodology. (5)

One obstacle to mounting a Daubert challenge to a differential etiology opinion is that many courts have already accepted the technique as a valid method for determining causation. (6) As set forth below, there should still be room to attack a differential etiology by arguing that the expert applied the technique in an unreliable and unscientific way. …

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