Vanderbilt Law Review

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In the decade since Daubert v. Merrell Dow Pharmaceuticals Inc., federal judges have exercised their role as gatekeepers of expert witness testimony to evaluate many different categories of scientific evidence. They have not done so without controversy, however. Because the element of causation in pharmaceutical product litigation is frequently dispositive, the application of Daubert to scientific evidence of causation has been particularly contentious. Plaintiffs in such cases must prove both general causation-that the product is capable of causing an injury of the type from which the plaintiff suffers-and specific causation-that the product was the actual cause of the plaintiffs injury. Daubert itself involved the admissibility of evidence of general causation in a pharmaceutical product liability action. Of particular interest here is Daubert's application to a common type of specific causation evidence known as differential diagnosis evidence. "Differential diagnosis" refers to the process by which a clinical physician, for purposes of treatment, identifies the condition affecting his patient and/or its cause. The admissibility of differential diagnosis evidence is crucial to a plaintiffs pharmaceutical product liability case, as testimony from a clinical physician that the drug product is the most likely cause of the plaintiffs condition is frequently the plaintiffs only means of proving specific causation.

Application of the Daubert standard to differential diagnosis testimony has engendered particular disagreement and confusion in the federal courts over the past several years. The recent series of cases surrounding Sandoz Pharmaceuticals Corporation's ("Sandoz") lactation-suppressing drug Parlodel aptly illustrates the uncertainty surrounding the admissibility of differential diagnosis evidence. After reports of a possible association between Parlodel and strokes, Sandoz withdrew the drug from the market as a lactation suppressant in 1994 at the request of the Food and Drug Administration. In the ensuing litigation, plaintiffs claimed that their ingestion of Parlodel caused them to suffer strokes." Courts have disagreed, however, on the reliability of the differential diagnosis evidence offered by these plaintiffs as part of their effort to prove causation. For example, on September 12, 2001, a federal district court judge in Illinois granted Sandoz's motion to exclude the expert opinions of two physicians prepared to testify based on a differential diagnosis that Parlodel caused the plaintiff to suffer a stroke several days after ingesting the drug. Nine days later, a federal magistrate in Alabama denied a similar motion brought by the same defendant regarding the same experts, the same drug, and the same type of injury in a suit brought by a different plaintiff." Purportedly applying the same admissibility standard-namely, Rule 702 of the Federal Rules of Evidence ("Rule 702" or "the Rule")-the federal court in Alabama found the proffered differential diagnosis testimony reliable and therefore admissible, while the federal court in Illinois had found it unreliable and therefore inadmissible.' In addition to excluding this crucial expert testimony on causation, the Illinois court granted the defendant leave to file a motion for summary judgment.' The Alabama case, on the other hand, proceeded to trial.