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Toxic tort cases dealing with latent diseases, such as leukemia and other cancers, frequently focus on risk measurement rather than on direct proof of causation. Unlike a traumatic injury or a disease clearly linked to a specific microorganism or virus, latent diseases often are of uncertain origin. Plaintiffs can claim only that an exposure to a product or environmental contaminant so increased the risk of the disease that the exposure more likely than not was a cause. As the Texas Supreme Court explained in Merrell Dow Pharmaceuticals Inc. v. Havner, 953 S.W.2d 706, 715 (1997), “In the absence of direct, scientifically reliable proof of causation, claimants may attempt to demonstrate that exposure to the substance at issue increases the risk of their particular injury.” A toxic tort plaintiff thus must show, at a minimum, that exposure to the contaminant or product at a given level and for a given time more than doubled the risk of injury. The theory of recovery is based on the “policy determination” that when the incidence of a disease is sufficiently elevated due to a toxic exposure, a purportedly exposed plaintiff can raise a fact issue as to causation. Because this theory depends on evidence of the relative incidence — or “relative risk” — of the disease at issue in exposed as compared to unexposed people, evidence of causation must come from epidemiologic studies. Epidemiology is the science that studies “the distribution and determinants of health-related states in specified populations.” John M. Last, ed., “A Dictionary of Epidemiology” 55 (3d ed. 1995). It is based on the commonsense proposition that if a substance causes a disease, then people exposed to the substance are more likely to contract the disease than are unexposed people. Epidemiologists compare different population groups to determine if there is any statistical association between a disease occurrence and some group characteristic, such as local environment, exposure to a particular substance, or personal living habits, such as cigarette smoking. Epidemiology focuses on the issue of general causation rather than on specific causation; that is, whether the alleged toxicant is capable of causing a disease, not whether it caused the disease in a particular person. Epidemiologic studies try to identify a statistical association between a suspected agent and disease. Researchers must then decide whether the association reflects true general causation. This still does not establish specific causation for a tort claim. The plaintiff must prove not only that the accused substance is capable of causing disease, but also that it did cause the plaintiff’s disease. THE LEGAL STANDARD FOR RISK-BASED PROOF Because epidemiologic studies address only risk, not specific causation, their use in litigation raises the question of how they relate to the legal “more likely than not” burden of proof. The Texas Supreme Court in Havnersought to harmonize the demands of science and legal causation by requiring that epidemiologic evidence demonstrate more than a “doubling” of the expected risk of disease. For example, if six cases of a disease occurred in an unexposed population of 1,000 persons and 25 cases occurred in 2,000 persons exposed to a toxicant, the statistical relative risk is 2.1. Exposed persons thus have more than double the risk of contracting the disease. While not a precise fit between the burdens of proof in science and law, this reasoning “strikes a balance between the needs of our legal system and the limits of science.” Havner, 953 S.W. 2d at 718. The “doubling” rule is actually a lesser standard than that applied by science. Most epidemiolgists require more than one study showing a relative risk of at least 3.0. Nonetheless, a substantial number of courts dealing with a variety of substances have followed the risk-doubling reasoning. Courts continue to struggle with the use of epidemiologic evidence, however. PARLODEL CASES ILLUSTRATE THE CURRENT CONFUSION Two recent federal district court cases, one from Alabama and one from Missouri, illustrate all too well the current confusion about the need for epidemiology and the need to distinguish general from specific causation. In Globetti v. Sandoz Pharmaceuticals Corp., 111 F. Supp. 2d 1174 (N.D. Ala. 2000), the plaintiff claimed her use of Parlodel — a drug prescribed to prevent lactation after a woman has given birth — had caused her to suffer a heart attack. The court noted that an epidemiologic study of this hypothesis was impossible because too few postpartum women have heart attacks or strokes to produce a statistically significant figure. Under such circumstances, the court suggested that a doctor’s differential diagnosis might suffice instead for legal purposes. “Science, like many other human endeavors, draws conclusions from circumstantial evidence when other, better forms of evidence are not available.” Id. at 1180. Another court, however, ruling on virtually the same evidence as in Globetti, held that a plaintiff cannot win such a case without epidemiologic evidence. In Glastetter v. Novartis Pharmaceuticals Corp., 107 F. Supp. 2d 1015 (E.D. Mo. 2000), the court noted that even the plaintiffs’ experts had acknowledged that epidemiology was the best way to prove causation. The court rejected the plaintiffs’ reliance on regulatory findings that Parlodel might cause heart attacks or strokes. Federal agencies operate from a preventive perspective, and when the utility and need for a drug are considered slight, they are especially willing to caution against its use if there is any hint of risk. DIFFERENTIAL DIAGNOSIS AND LEGAL CAUSATION The Globetticourt’s heavy reliance on the use of “differential diagnosis” by the plaintiffs’ experts raises serious questions. This methodology is generally used to determine what a patient suffers from, not the cause of the patient’s condition. To the extent the logic of differential diagnosis is used to determine causation, it requires a physician to list the possible known causes of a patient’s condition, then use applicable diagnostic tests to eliminate causes until he or she is left with the most likely. In Globetti, the plaintiff’s physicians purportedly used various tests, including physical examination, blood testing and X-rays, to isolate vascular “spasm” as the only possible cause of her heart attack. Since Parlodel has “vasoconstrictive characteristics,” they opined that it was the most likely agent. The problem here is that the Globettiexperts skipped a step in their analysis. Before a factor may be included in the list of possible causes for the differential diagnosis, there must be evidence that it is capable of causing the injury or effect. A fundamental assumption underlying the method of differential diagnosis “is that the final, suspected ’cause’ remaining after this process of elimination must actually be capable of causing the injury.” Cavallo v. Star Enterprise, 892 F. Supp. 756, 771 (E.D. Va. 1995), modified, 100 F.3d 1150 (4th Cir. 1996). As the Glastettercourt recognized, an expert must come forward with the evidence “ruling in” Parlodel as a potential cause, as well as ruling out other causes. Glastetter, 107 F. Supp. 2d at 1028. Without evidence of general causation, a differential diagnosis is untenable, and should be inadmissible under the U.S. Supreme Court’s seminal ruling in Daubert v. Merrell Dow Pharmaceu-ticals Inc., 509 U.S. 579 (1993). EPIDEMIOLOGY AND THE ‘RARE’ DISEASE Another issue raised by the Globetticourt has become a frequent point of controversy: How can epidemiology address a disease or injury that rarely occurs? The Globettiplaintiffs argued successfully that epidemiologic evidence was not practical because the occurrence of myocardial infarctions among postpartum women is rare — a rate of 1 to 1.5 per 100,000. Globetti, 111 F. Supp. 2d at 1179. That no evidence yet exists, however, is not legal proof of causation. The courtroom is not the place for scientific guesswork. “Law lags science; it does not lead it.” Rosen v. Ciba-Geigy Corp., 78 F.3d 316, 319 (7th Cir.), cert. denied, 519 U.S. 819 (1996). A similar argument was recently rejected by the Texarkana court of appeals in Austin v. Kerr-McGee Refining Corp., 25 S.W.3d 280 (Tex. App.-Texarkana 2000, no writ). The plaintiff in Austin contracted chronic myelogenous leukemia (CML), which plaintiffs claimed was due to workplace exposure to benzene. Plaintiffs conceded that there was insufficient data to establish an association between benzene and CML, but asserted that because CML is relatively rare, “there are simply not enough cases of the disease for epidemiologists to generate a statistically significant relative risk.” 25 S.W. 3d at 291. But this lack of evidence goes to the heart of the tort system, which is not a generator of recovery without proof. “Our legal system requires that claimants prove their cases by a preponderance of the evidence.” Id. at 292, quoting Havner, 953 S.W.2d at 728. This concern was recognized in Daubert, which explicitly noted that gatekeeping may prevent a jury “from learning of authentic insights and innovations.” 509 U.S. at 597. On the other hand, there is a converse unfairness under an evidentiary scheme that allows recovery to everyone who claims some injury and some exposure. Rigorous judicial scrutiny, in toxic torts requiring reliable epidemiological evidence, is essential to a fair determination of liability. As Justice Stephen Breyer wrote in concurrence in General Electric Co. v. Joiner, 118 S.Ct. 512, 520 (1997), such gatekeeping assures “that the powerful engine of tort liability, which can generate strong financial incentives to reduce, or to eliminate, production, points toward the right substances and does not destroy the wrong ones.” Clifton T. Hutchinson is a partner with Hughes & Lucein Dallas. He can be reached at (214) 939-5500.

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