October 19, 2006
Article originally published in The American Bar Association newsletter, TortSource:A Publication of the Tort Trial & Insurance Practice Section, Vol. 8, No. 4 Summer 2006
Most courts recognize that "law lags science; it does not lead it." Rosen v. Ciba-Geigy Corp., 78 F.3d 316, 319 (7th Cir. 1996). Nonetheless, injured parties often enter the courtroom armed with theories of causation that the medical or scientific communities have not verified.
For example, a hypothetical patient with symptoms of fibromyalgia argues that his condition was caused by falling on a slippery floor even when the medical community does not recognize a connection between physical trauma and fibromyalgia. To clear this evidentiary hurdle, the patient's physician cites case reports in which clinicians have observed symptoms of fibromyalgia in patients after they experienced physical trauma. The expert examines the patient and uses the "differential diagnosis" method to determine that there are no other likely causes for the condition.
At trial, and over the defendant's objection, the expert testifies that the fall caused the fibromyalgia. Recent case law confirms that the expert's use of the differential diagnosis methodology to prove general causation should be excluded under Daubert v. Merrell Dow Pharmaceutical, Inc., 509 U.S. 579 (1993), or in jurisdictions where the Frye standard is employed. See Frye v. United States, 239 Fed. 1013 (D.C. Cir. 1923).
The Case Law Excluding Differential Diagnosis
In McClain v. Metabolife International, Inc., 401 F.3d 1233 (11th Cir. 2005), the Eleventh Circuit Court of Appeals applied Daubert to reverse the trial court's admission of differential diagnosis testimony because it was not based on reliable proof of general causation. In its analysis, the court defined differential diagnosis as used in both medical and legal contexts:
Differential diagnosis involves the determination of which one of two or more diseases or conditions a patient is suffering from, by systematically comparing and contrasting their clinical findings. This leads to the diagnosis of the patient's conditions, not necessarily the cause of that condition. The more precise but rarely used term is differential etiology, which is a term used on occasion by expert witnesses or courts to describe the investigation and reasoning that leads to the determination of external causation, sometimes more specifically described by the witness or court as identifying external causes by a process of elimination.
McClain v. Metabolife International, Inc. at 1252 (internal citations omitted).
The McClain plaintiffs claimed they suffered ischemic strokes and an acute myocardial infarction due to ingesting Metabolife, an herbal appetite suppressant containing ephedrine and caffeine. The medical community did not generally recognize the toxicity of this chemical combination. At trial, the plaintiffs provided expert opinion supporting the general toxicity of Metabolife based on analogies, case studies, and other methods. The experts reached their conclusions as to the specific cause of the plaintiffs' ailments using the differential diagnosis method. They ruled in each potential cause of the plaintiffs' injuries and included Metabolife, based on their opinion of its general toxicity. They then obtained medical histories, performed clinical examinations, and ruled out each typical cause of the injuries except for Metabolife ingestion.
In framing its analysis, the Eleventh Circuit noted that establishing medical causation requires proof of both general and specific causation. It defined general causation as addressing whether an agent increases the incidence of a particular harm in the population. This evaluation is of critical importance when "the medical community does not generally recognize the agent as capable of causing the plaintiff's injury." Id. at 1239. Specific causation asks whether the claimant's injuries were, in fact, due to the external agent.
The court found the methodology used by the experts to support Metabolife's general toxicity was not reliable and should have been excluded under Daubert. It then summarily rejected the experts' differential diagnosis, holding that "a valid differential diagnosis ... only satisfies a Daubert analysis if the expert can show the general toxicity of the drug by reliable methods." Id. at 1253. Additionally, an expert's conclusions are not made dependable under Daubert simply because a differential diagnosis was performed. The court recognized the principle that general causation must be proved by reliable methods prior to determining the specific cause of an injury by differential diagnosis. A differential diagnosis performed without doing so lacks foundation and is not reliable.
Similarly, in Norris v. Baxter Healthcare Corp., 397 F.3d 878 (10th Cir. 2005), the Tenth Circuit Court of Appeals upheld the exclusion of differential diagnosis testimony when the plaintiffs' expert failed to provide a solid basis for general causation and did not address contrary epidemiology finding no causal link between breast implants and systemic disease. The court held that epidemiological evidence is often the best evidence of general causation and that the plaintiffs' dependence on case studies did not provide a reliable basis for general toxicity.
Differential Diagnosis after Proving General Causation
In contrast, the admissibility of causation opinions based on differential diagnosis has been affirmed when the expert first presents credible evidence of general causation. Clausen v. M/V New Carissa, 339 F.3d 1049 (9th Cir. 2003), involved an oil spill that allegedly caused the death of oysters the plaintiffs seeded in an estuary. No literature directly supported the theory advanced by the plaintiffs' expert that the oysters perished due to low-level contact toxicity with oil particulates. The expert determined this specific cause by examining the facts and eliminating as unlikely each of five alternative potential causes of the oysters' mortality. On appeal, the defendants argued that the expert's ruling in of contact toxicity was not reliable because the theory lacked support in the literature.
In upholding the admission of the testimony, the Ninth Circuit Court of Appeals indicated that the first step in conducting a differential diagnosis is to compile a comprehensive list of hypotheses generally capable of causing the patient's symptoms. Further, an expert's decision to rule in a potential cause is not necessarily dependent on whether a cause-effect relationship has been conclusively established in the literature or whether the opinion was produced independent of litigation. The expert may conduct litigation-based research to show general causation as long as the methods used are reliable.
The court held that the plaintiffs' expert's decision to rule in contact toxicity was reliable because it was "based on a variety of objective, verifiable evidence." Id. at 1060-61. The expert performed a histopathological examination, cited data in government reports, took into account temporal and geographic considerations, and made proper analogies to other animal systems. Thus, even in the absence of literature directly on point, the expert's general causation opinion was reliable and provided a proper foundation for differential diagnosis.
The court further explained that while the expert's theory was not specifically established in shellfish, it had been documented in published studies in all other tested animals. It was also significant that the defendants' expert ruled in contact toxicity and had written a paper on the subject, thereby providing further support for the plaintiff's theory of general causation. Finally, the court sympathized with the lack of literature because it noted that the opportunity to study the effects of oil spills is rare.
The differential diagnosis methodology is sometimes misused when litigants attempt to explain the currently "unexplainable" in the courtroom. However, expert causation opinions based on this approach are inadmissible under Daubert when used to prove general causation. This principle is also significant in states employing Frye because using differential diagnosis to prove general causation is not a generally accepted methodology. Absent proof of general causation, a differential diagnosis is irrelevant and cannot assist the trier of fact.
Charles A. Alfonzo is a member of Burnham Brown PLC, located in Oakland, California. Rohit A. Sabnis is an partner with the same firm. The authors concentrate their practices in all aspects of civil litigation. They can be reached, respectively, at firstname.lastname@example.org, email@example.com