A federal court recently granted the defendant’s Daubert motion, excluding plaintiff’s causation expert in a case alleging that defendant’s chemical caused plaintiff’s non-Hodgkins Lymphoma. Pritchard v. Dow Agro Sciences, et al., 2010 U.S. Dist. LEXIS 23098 (W.D. Pa. March 11, 2010). The case is illustrative of three main points.
But first, the background. In this toxic tort case, Mr. Pritchard and his wife alleged that chemicals manufactured and sold by defendants caused Mr. Pritchard to develop NHL. Plaintiff asserted claims of negligence and strict liability. In particular, the dispute surrounded whether Mr. Pritchard’s exposure to Dursban was the legal cause of his disease. Dursban is the trade name for a group of insecticide products containing chlorpyrifos as the active ingredient. The etiologies or causes of NHL and plaintiff’s subtype, are generally unknown. Several risk factors have been identified, including: people with severe immunodeficiency, human immunodeficiency virus (HIV), human T-cell leukemia virus type 1, hepatitis C virus; a family history of NHL; and occupational exposure to herbicides, and certain other chemicals, said the court.
In support of their claim that Mr. Pritchard’s exposure to Dursban caused his NHL, plaintiffs identified Dr. Bennet Omalu as their expert on medical causation and submitted his expert report. Defendants filed a Motion to Exclude the Expert Causation Testimony of Dr. Omalu, attaching rebuttal experts reports. The court held a Daubert hearing during which the parties presented no additional evidence, only arguing the legal issues in dispute. Under the federal rules, the district court acts as a gatekeeper, preventing opinion testimony that does not meet the requirements of qualification, reliability and fit from reaching the jury. In this role, the district court must satisfy itself that good grounds exist for the expert’s opinion.
The opinions. Dr. Omalu concluded that Mr. Pritchard’s prolonged occupational exposure to Dursban insecticides significantly contributed to the patho-etiology of his Non-Hodgkin’s Lymphoma. He described the general risk of exposure to pesticides to humans, stating that these chemicals are highly toxic and dangerous and have been identified as human carcinogens. He further opined that no level of exposure to agricultural chemicals should be deemed safe. The expert testified at his deposition that he had considered and analyzed other potential causes of Mr. Pritchard’s Non-Hodgkins Lymphoma, ruling out those other causes that were not sufficient to be a sole cause of the disease. Dr. Omalu also testified that diabetes may have contributed to Mr. Pritchard’s disease but that it was “less likely to have contributed” and that Mr. Pritchard’s obesity and body mass index were not highly associated with NHL.
The challenge. Defendants attacked the reliability of Dr. Omalu’s proffered testimony, arguing that his opinions did not reliably establish general causation, i.e., that exposure can cause NHL, or specific causation, i.e., that Mr. Pritchard’s exposure to Dursban caused his NHL. Courts in toxic tort cases often separate the causation inquiry into general causation -whether the substance is capable of causing the observed harm in general- and specific causation -whether the substance actually caused the harm a particular individual suffered. E.g., Perry v. Novartis Pharmaceuticals Corp., 564 F.Supp.2d 452, 463 (E.D.Pa. 2008).
General causation. General causation is often established in a toxic tort case through the use of epidemiological studies. Epidemiology is ‘the primary generally accepted methodology for demonstrating a causal relation between a chemical compound and a set of symptoms or a disease. E.g., Soldo v. Sandoz Pharmaceuticals Corp., 244 F.Supp.2d 434, 532 (W.D.Pa. 2003); Conde v. Velsicol Chem. Corp., 804 F.Supp. 972, 1025-26 (S.D.Ohio 1992); see also Perry, 564 F.Supp.2d at 465. Epidemiological studies examine the pattern of disease in human populations, General Elec. Co. v. Joiner, 522 U.S. 136, 144 n.2, (1997), and, more specifically, epidemiological evidence identifies agents that are associated with an increased risk of disease in groups of individuals, quantifies the amount of excess disease that is associated with an agent, and provides a profile of the type of individual who is likely to contract a disease after being exposed to an agent. Federal Judicial Center, REFERENCE MANUAL ON SCIENTIFIC EVIDENCE, Reference Guide on Epidemiology at 335-36 (2d ed. 2000).
The Third Circuit has recognized that epidemiology is a well-established branch of science, and epidemiological evidence has been accepted in numerous cases. The parties here agreed that general causation can be established through the use of such studies. (Of course, the leap from association shown by such studies to a causation conclusion is a judgment based on the assessment of the strength, power, and plausibility of the studies, among other factors.) However, defendants challenged Dr. Omalu’s general causation opinion that exposure to Dursban can cause NHL because his opinions did not rely on statistically significant epidemiological studies, and that Dr. Omalu did not rely on an epidemiological study showing an association between chlorpyrifos and NHL with a relative risk greater than 2.0. They also argued that Dr. Omalu’s opinion was unreliable because he ignored published epidemiology studies which show a lack of an association or an inverse relationship between chlorpyrifos and NHL.
Plaintiffs contended that they were not required to present statistically significant studies which demonstrate an association between chlorpyrifos and NHL in order to demonstrate general causation. The court found that the law regarding the need for such epidemiological studies is not clearly established; therefore, the court was hesitant to adopt per se rules of admissibility but should consider the following as factors in determining the reliability of an expert’s opinion on general causation: whether the expert relied on epidemiological studies; whether the expert ignored or sufficiently addressed epidemiological studies which contradicted his hypothesis, explaining the discrepancy between his hypothesis and that of the authors; and, whether the findings set forth in the studies are statistically significant. Clearly if epidemiology studies are not available, an expert must offer a reliable causation opinion through the use of some other valid scientific methodology.
On the issue of relative risk, some courts have refused to consider epidemiology studies with less than a relative risk of 2.0 as supporting an association between a chemical agent and a disease. See Daubert v. Merrell Dow Pharmaceuticals, Inc., 43 F.3d 1311, 1320 (9th Cir.), cert denied, 516 U.S. 869 (1995); see also In re W.R. Grace & Co., 355 B.R. 462, 482 (D. Del. Bankr. 2006)(adopting 2.0 relative risk standard). Other courts have permitted the use of studies showing less than a 2.0 relative risk in support of a causation opinion. See In re Joint E. & S. Dist. Asbestos Litig., 964 F.2d 92, 97 (2d Cir. 1992). But see DeLuca by DeLuca v. Merrell Dow Pharmaceuticals, 911 F.2d 941, 958-59 (3d Cir. 1990)(if New Jersey law requires plaintiffs to show that it is more likely than not that Bendectin caused the injury, and if plaintiffs rely solely on epidemiological analysis in order to avoid summary judgment, the relative risk from the epidemiological data relied upon will, at a minimum, have to exceed 2). Here the court decided to follow the approach taken in Magistrini v. One Hour Martinizing Dry Cleaning, 180 F.Supp.2d 584, 606 (D.N.J. 2002), where the district court explained that a relative risk of 2.0 is not so much a password to a finding of causation as one key piece of evidence, among others for the court to consider in determining whether an expert has employed a sound methodology in reaching his or her conclusion.
The court found that the expert had not relied on statistically significant studies. Some were clearly not significant, while on others there was no evidence of record explaining the method that Dr. Omalu used. Plaintiffs’ counsel’s heavy involvement in drafting the expert declaration further undermined the reliability of the opinions contained therein. (Plaintiffs’ counsel admitted that he drafted the declaration, and the expert then signed it without any changes.)
On the issue of the admissibility of expert testimony that fails to account for published epidemiology studies with conclusions contrary to the expert’s opinion, the court looked at Perry v. Novartis Pharmaceuticals Corp., 564 F. Supp.2d 452 (E.D.Pa. 2008). In Perry, the district court recognized that “no reliable scientific approach can simply ignore the epidemiology that exists.” Perry, 564 F.Supp.2d at 465. Dr. Omalu never addressed two studies raised by defendants. Dr. Omalu offered no explanation regarding the studies’ findings and his hypothesis in this case.
Thus in sum, Dr. Omalu’s general causation opinion that exposure to Dursban could cause NHL was found unreliable. Plaintiffs could not rely on Dr. Omalu’s bare assertions that “studies” show that there is an association.
Specific Causation. The court then turned to the parties’ respective positions on Dr. Omalu’s specific causation opinions that Mr. Pritchard’s exposure to Dursban caused his NHL. Plaintiffs, as is typical, relied on a so-called differential diagnosis. The use of differential diagnosis is “the basic method of internal medicine,” and is generally accepted for diagnosing a patient. The process rules in potential explanations for the patient’s symptoms, and then rules each disease out until a final diagnosis remains. More controversial is its use to explain the cause of the disease. But here the district court notes that it has been accepted in some courts as a technique that involves assessing causation with respect to a particular individual. “Differential diagnosis, or differential etiology, is a standard scientific technique which identifies the cause of a medical problem by eliminating the likely causes until the most probable one is isolated.” Magistrini, 180 F.Supp.2d at 609. Although the use of differential diagnosis is generally accepted, said the court, an expert’s use of same must still be evaluated in order to ensure that the methods employed are reliable. The mere statement by an expert that he or she applied differential diagnosis in determining causation does not ipso facto make that application scientifically reliable or admissible.
Here, defendants contended that Dr. Omalu’s differential diagnosis was unreliable because of his limited review of Mr. Pritchard’s case, not physically examining Mr. Pritchard, not reviewing all of his medical records and relying, in part, on an oral medical history transmitted to others. The district court concluded that the expert’s failure to review all of Mr. Pritchard’s medical records, his discovery responses, his deposition testimony, his application records, or any other evidence regarding Mr. Pritchard’s exposure to chlorpyrifos or other pesticides, left his opinions unsupported and rendered his differential diagnosis methodology unreliable.
While an expert may not always have to exclude all other theoretically possible causes, an expert surely must rule out obvious alternative causes, and also address the plausible alternative causes proposed by a defendant. It is necessary for the plaintiff’s expert to offer a good explanation as to why his or her conclusion remains reliable in the face of plausible alternatives. Significantly, Dr. Omalu did not attempt to rule out that Mr. Pritchard’s NHL is the result of an idiopathic or unknown cause despite the fact that the general cause of NHL is unknown. Standing alone, the presence of a known risk factor is not sufficient basis for ruling out idiopathic origin in a particular case, particularly where most cases of the disease have no known cause. Likewise, Dr. Omalu’s conclusion that Mr. Pritchard’s NHL is not idiopathic because he determined that his disease was caused by exposure to Dursban, was not a sufficient explanation, as it is merely a circular conclusion.
Fit. The court also found that the conclusion reached by Dr. Omalu did not reliably flow from the data and methodology used in this case. For one, the data was flawed as Dr. Omalu failed to properly consider all of the relevant materials, specifically, Mr. Pritchard’s medical records, as well as all the exposure information.
Finally, plaintiffs also raised a novel argument as an alternative to the defendants’ challenges based on Federal Rule of Evidence 702 and Daubert and its progeny. They contended that the application of several principles argued by the defendants, if applied to exclude Dr. Omalu’s testimony, would violate the Erie doctrine. (Yes, we are going back to first year civil procedure.) The Erie doctrine provides that a federal court sitting in diversity must apply state substantive law and federal procedural law. In other words, the application of Daubert principles were inconsistent with state substantive law on causation. The court noted first that the expert’s proffered testimony was not being excluded solely on the challenged bases, which were considered among a host of other deficiencies in Dr. Omalu’s proffered testimony. The court did not adopt any “bright-line” rules but instead evaluated the testimony using a “flexible” approach. In any event, the court briefly addressed the contention and concluded that state substantive law does not change the federal standard for the admissibility of expert testimony. Federal Rule of Evidence 702 could and must be applied to this action.