Differential diagnosis and Daubert: preventing the misuse of differential etiology to prove causation in toxic tort cases.

• Author: Hopp, Anthony G.

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.

1. 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.

   1. 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.

   2. Joiner and the "analytical gap"

      The Supreme Court's decision in General Electric v. Joiner dealt with extrapolation from animal studies to human results. (7) The plaintiff's experts had relied on "relevant animal studies which support their opinions" and a handful of human epidemiology studies to conclude that PCBs caused plaintiff's lung cancer. (8) The Supreme Court observed that the animal studies were on infant mice who had received direct injections of high doses of PCBs, whereas the plaintiff was an adult male who had been exposed to much lower concentrations. The mice also developed a different form of cancer.

      Plaintiff defended his experts' opinions by claiming that, under Daubert, the court's focus should be on "principles and methodology" and not on the experts' conclusions. The Supreme Court responded with perhaps the most widely quoted language in all of Daubert jurisprudence:

      But conclusions and methodology are not entirely distinct from one another. Trained experts commonly extrapolate from existing data. But nothing in either Daubert or the Federal Rules of Evidence requires a district court to admit opinion evidence that is connected to existing data only by the ipse dixit of the expert. A court may conclude that there is simply too great an analytical gap between the data and the opinion proffered. (9) Joiner both clarified and complicated the district court's gatekeeper role. It freed district courts from focusing exclusively on an expert's methodology in determining whether opinions are admissible, but also saddled the court with the sometimes murky responsibility of determining whether the "analytical gap" between an expert's source material and his or her conclusions is "too wide." After Joiner, district courts were free to review epidemiological studies, animal studies, in vitro studies and other forms of scientific proof in order to evaluate whether the relevant scientific studies supported the experts' conclusions. (10)

      Joiner provided no guidance for the lower courts on how to tell when "the gap" was "too great." Ever since, district courts have generally observed that the Daubert/Joiner analysis is fact specific and case-by-case. (11) While some courts merely seek to evaluate the "weight" of the scientific evidence without closely examining that evidence, other courts have rolled up their sleeves and dug into the often-unfamiliar territories of toxicology, epidemiology and animal research at a granular level. (12) The Seventh Circuit recently described the analysis this way: "The district court's primary concern, and ours on appeal, is the failure of the experts to connect the dots from the studies to the illnesses endured by the [plaintiffs]." (13) The Joiner/ analytical gap assault is another potential strategy for keeping a speculative differential etiology opinion out of evidence. The defense lawyer can argue that the dots do not connect because the gap between the evidence on which the expert purports to rely and the ultimate result is too wide.

   3. The Frye standard: General acceptance

      In jurisdictions that follow the Frye standard, the court's focus is on whether the expert's theory is generally accepted in the relevant scientific community. (14) Even in Frye jurisdictions, many courts have added a reliability test. That is, the technique the expert applies must give rise to reliable results and the expert must have applied the technique in a reliable manner. The addition of the "reliability" element in Frye jurisdictions gives courts wider latitude to exclude expert opinions, and some courts have applied the "Frye plus reliability" standard to exclude differential etiology opinions. (15)

2. Differential Diagnosis vs.

   Differential Etiology

   The terms differential diagnosis and differential etiology sound similar, but they describe vastly different processes. The Federal Judicial Center's Manual on

   Scientific Evidence defines the term "differential diagnosis" as:

   The method by which a physician determines what disease process has caused a patient's symptoms. The physician considers all relevant potential causes of the symptoms and then eliminates alternative causes based on a physical examination, clinical tests and a thorough case history. (16) Somewhat paradoxically, even courts that have allowed plaintiff's experts to testify to a differential etiology have recognized that a differential diagnosis is a technique used to identify, i.e., diagnose, the plaintiff's illness, not to determine the cause of the plaintiff's illness. (17)

   For example, in Tamraz v. Lincoln Electric Company, (18) the court criticized the plaintiff's expert for conflating diagnosis and etiology. The plaintiff had been exposed to manganese, but developed Parkinson's, not manganism. Manganism is caused by exposure to manganese, Parkinson's is not. The two conditions share similar symptoms, but arise from damage in different parts of the brain. The plaintiff's expert called the plaintiff's illness "manganese-induced parkinsonism," and attempted to "elide[ ] the distinction between [plaintiff's] disease and what caused it." (19) The Sixth Circuit excluded the opinion, but still held that a differential etiology--when done correctly--is an admissible method for proving causation.

   The court noted that, when most physicians think about etiology in a clinical setting, they may think about it in a different way than judges and juries think about it in a courtroom. Getting a diagnosis right matters greatly to a treating physician, as a bungled diagnosis can lead to an unnecessary procedure, or worse. But with etiology, the physician may follow a precautionary approach. If a particular factor might cause the disease, the physician may tell the patient to avoid it. In other words, according to the Sixth Circuit, physicians apply a less stringent standard to evaluating etiology than they do to diagnosing an illness. The Sixth Circuit went on to observe that in its own prior opinions, (20) it had...