Daubert, doctors and differential diagnosis: treating medical causation testimony as evidence.

• Author: Kent, Jr., Michael B.

An assessment of admissibility is not the same as an assessment of sufficiency, but Daubert has created that confusion

CAROL HELLER began experiencing various respiratory problems shortly after Shaw Industries installed new carpeting in her house.(1) A few months later, she consulted an allergist, Dr. Joseph Papano. He took her medical and family

histories, questioned her about the house's environment, ordered chest x-rays, and performed several laboratory tests. Based on his findings from the collected data, Dr. Papano ruled out various possible causes of Heller's respiratory problems. This winnowing process, known as "differential diagnosis" in the medical community, coupled with the close temporal relationship between Heller's symptoms and the installation of the new carpet, led Dr. Papano to conclude that the carpet was the cause of her illness.

Heller sued Shaw Industries for, among other things, failure to warn and defective design. She called Dr. Papano as an expert witness to testify on the issue of causation, but the trial court excluded his testimony. The problem: Although Dr. Papano conducted a differential diagnosis to determine what did not cause Heller's symptoms, he could point to no studies indicating that the carpet could cause them. Without this testimony, Heller could not prove causation, and the court granted summary judgment in favor of Shaw.

The Third Circuit, while disagreeing with some of the trial court's rationale, affirmed both the exclusion of Dr. Papano's testimony and summary disposition in favor of Shaw.

THE PROBLEM

The Heller case reveals one of the central problems underlying most mass and toxic tort litigation--proving that the defendant's product caused the plaintiff's injury. As is the case with other torts, the plaintiff in a mass or toxic tort case must show duty, negligence, causation and damage. In the mass or toxic tort context, however, the element of causation often plays a dispositive role. Plaintiffs must prove both that the product is capable of producing the injuries (general causation) and that the product actually did so (specific causation).(2) To do so, they must rely on the testimony of experts--toxicologists, engineers, physicians--with scientific and technical knowledge of the product in question and its effects on the human body. If the testimony of these experts is either inadmissible or insufficient to carry the burden of proof, plaintiffs' cases cannot go forward. Accordingly, the admissibility and sufficiency of the expert testimony often becomes a hotly contested issue.

Inherent in ruling on the admissibility of expert testimony is that courts must review the underlying science to determine the reliability and relevance of the evidence. For 70 years, the "general acceptance" test from Frye v. United States(3) enjoyed almost uniform application in both state and federal courts.(4) This test admitted scientific evidence if, and only if, it was generally accepted in the pertinent scientific community. Under Frye, courts looked deferentially to scientists in determining whether the proffered evidence met the standards of scientific reliability.

This era of deference came to an end with the Supreme Court's decision in Daubert v. Merrell Dow Pharmaceuticals Inc.,(5) which interpreted Federal Rule of Evidence 702 as superseding the common law "general acceptance" test. The Court read Rule 702 as establishing a system wherein federal trial judges ensure the reliability, as well as the relevance, of scientific testimony.

Trial judges must serve as "gatekeepers" when it comes to scientific evidence, assessing the methodologies underlying the testimony of expert witnesses. Trial judges must review and screen evidence based on everything from aerodynamics to epidemiology. The "gatekeeper" role was reaffirmed in General Electric Co. v. Joiner(6) and Kumho Tire Co. v. Carmichael.(7)

A source of scientific evidence in mass and toxic tort litigation is the medical technique called differential diagnosis. It is a clinical procedure whereby medical doctors determine which of several potential diseases is causing the patient's symptoms by ruling out possible causes until only one or two remain.(8)

Several courts have recognized the significance of this procedure to the causation issue.(9) The problem thus turns on the reliability and fit of using the technique to determine cause in the legal, rather than the clinical, sense. Although the cases have fleshed out many of the questions concerning the use of differential diagnosis as evidence of causation, the answers to these questions remain somewhat vague because courts often confuse general and specific causation, as well as admissibility and sufficiency. A clear treatment of the issue is needed.

DIFFERENTIAL DIAGNOSIS

Differential diagnosis, as used in the medical profession, is a clinical process whereby doctors determine from what disease a patient suffers. By comparing the patient's symptoms to symptoms associated with known diseases, the physician attempts to identify the disease or diseases that best explain the facts of the patient's case.(10) Identification takes place through a process of elimination,(11) with the physician collecting data on the patient's history and illness, analyzing that data and ruling out various diseases until a final diagnosis is reached. In short, differential diagnosis is the act of distinguishing one disease from another to select a proper treatment.(12)

It thus aids a physician in determining the injury from which a person suffers, but the technique also is presented as evidence of causation at trial. This has drawn criticism from courts and commentators alike.(13) This criticism often is justified because, while physicians speak in terms of "cause," they generally focus their differential diagnosis on defining the patient's illness.(14)

On the other hand, a thorough diagnosis frequently considers the underlying causal agents of a disease in prescribing a treatment.(15) Indeed, some courts describe the technique of differential diagnosis as "differential etiology," a term that heavily stresses the causation issue since etiology is the science and study of the causes of disease.(16)

When physicians seek to determine the disease causing the symptoms--for example, lung cancer--they often look for and rule out known etiologic agents of the disease--for example, asbestos. The most likely agent remaining after this sifting is considered the cause of the disease. If no known etiologic agents remain on the list, attention is focused on any suspicious substances, such as chemical compounds, to which the patient has been exposed. For treatment purposes, one or more of these substances is presumed to be the cause of the patient's disease.(17)

Therefore, differential diagnosis often consists of a clinical determination, by process of elimination, of both the disease causing the patient's symptoms and the most likely etiologic agent causing that disease. The problem arises from extrapolating from this determination of clinical causation, for treatment purposes, to causation in the legal sense.

Because physicians utilize differential diagnosis in a clinical setting, some lawyers contend that it does not constitute science.(18) Medical writers, however, indicate the contrary, noting that a physician should perform a differential diagnosis in the same manner as other scientific research--objectively collecting all the facts, analyzing them in an unprejudiced fashion and ending with a logical conclusion.(19)

Certainly, the steps in conducting a proper differential diagnosis resemble the scientific method, and differential diagnosis remains scientific in nature although performed in a clinical environment. Accordingly, the Supreme Court's decision in Daubert applies to its use as causation evidence in federal trials.

DAUBERT AND JOINER

1. Daubert

   Daubert is the seminal case regarding the admissibility of scientific expert testimony. The new system it instituted for testing the admissibility of scientific evidence has resulted in confusion among commentators and the lower courts.(20)

   The Court held that Rule 702 of the Federal Rules of Evidence supersedes the common law Frye test of general acceptance in the relevant scientific field. The Court first noted that the Federal Rules render a broad range of evidence admissible and that neither Rule 702 nor its legislative history mentions "general acceptance." The Court concluded that Congress did not intend the "general acceptance" standard to be applied as the sole test of admissibility.

   Having determined that Frye no longer applied to litigation in the federal system, the Court declared that Rule 702 establishes a "gatekeeping" role for the federal trial courts, obligating the judge to "ensure that any and all scientific testimony or evidence is not only relevant, but reliable."

   As to reliability, the Court noted that Rule 702 required that an expert's testimony be "scientific knowledge," a term that implies a grounding in the procedures of science. "Knowledge" implies that the proposition must constitute more than subjective belief or unsupported speculation. Together, the two terms mean that experts must derive their assertions by means of the scientific method.

   As to relevancy, the Court also looked to the language of Rule 702, noting that the rule requires that the scientific evidence assist the trier of fact. To be admissible, the evidence must "fit" the factual dispute. "Fit" is not always obvious, the Court explained, as scientific validity for one purpose does not necessarily constitute scientific validity for another. But there must be a scientific connection between the testimony and the pertinent inquiry.

   To help the trial courts determine whether evidence constitutes scientific knowledge that will assist the trier of fact, the Court articulated the following nonexhaustive list of factors: (1) whether the theory or technique...