Expert Witnesses Not Required to Rule Out All Possible Causes in Product Liability Actions

On June 6, 2014, the 8th Circuit Court of Appeals reversed a district court decision excluding three experts, who were critical to the plaintiff to show causation, on the basis that they did not reliably rule out alternative causes of injury. The Court of Appeals found that even without ruling out all possible alternative explanations, the experts' opinions were reliable enough to assist the trier of fact.

In the case, a guardian ad litem brought product liability claims against Mead Johnson & Company, alleging that an infant contracted meningitis and suffered severe brain injuries due to the consumption of a contaminated powdered infant formula. The plaintiff's experts purported to conduct a differential diagnosis, ruling in the formula as a potential cause of the illness and ruling out other possible causes. However, in its Daubert motions, Mead Johnson & Company argued that such differential diagnoses were severely flawed or nonexistent with regard to certain significant issues. The district court agreed in its subsequent opinion, and was critical of the experts, finding they overlooked or did not test for evidence contrary to their conclusions and ruled out other potential causes of illness based on "unreliable methodology." The district court excluded plaintiff's experts' testimony in whole and granted summary judgment.

In reversing the district court's decision, the Court of Appeals determined that Daubert and Federal Rule of Evidence 702, which control the admission of expert testimony, served to liberalize the admission of expert testimony (Daubert v. Merrell Dow Pharmaceuticals, Inc.,509 U.S. 579 (1993)). Ultimately, it found that the district court violated the liberal admission standard, suggesting that doubts should be resolved in favor of admission. The Court of Appeals distinguished the instant case from precedent, Glastetter, relied upon by the district court, finding that while the Glastetter expert used reliable differential diagnosis methodology, they had no scientific proof of causation, rendering their opinion scientifically invalid (Glastetter v. Novartis Pharmaceuticals Corp., 252 F.3d 986 (8th Cir. 2001)). The district court, it concluded, went too far by weighing the experts' conclusions.

Ultimately, the Court of Appeals sent a clear message that experts are not required to consider all possible causes in conducting a differential diagnosis, and further, that because ruling out particular causes is an area where experts might reasonably differ, the jury should decide among conflicting opinions.

The 8th Circuit's "liberal admission" language and analysis of how a differential diagnosis may or may not be challenged is certainly at odds with dozens of other rulings across the country by district courts and other circuit courts that have used Daubert and its progeny to exclude expert opinions based on faulty or non-existent differential diagnoses. However, practitioners in the 8th Circuit at least should take note that conclusory differential diagnoses may not be so easily challenged, and reliability of the same will be left to the jury to weigh.