The Sixth Circuit has long been a proponent of differential diagnosis for the establishment of specific causation in toxic tort cases. It recently expanded the application of differential diagnosis to establish general causation in these cases. In doing so, the Sixth Circuit struck an injurious blow to defendants by making it more likely that a plaintiff may recover damages for injuries. In short, while a plaintiff must still meet his burden of proving his case beyond a preponderance of the evidence, the Sixth Circuit's recent decisions make it easier for a plaintiff to meet this burden, thus making it more likely that a plaintiff will be awarded damages.

Differential diagnosis is defined as a method in which a physician or medical expert determines what disease and/or exposure process caused a patient's symptoms. Physicians and medical experts use this method to help determine the cause of a patient's alleged injury. In making a determination of what ails a patient, a physician considers all relevant potential causes of the symptoms (i.e., reviewing physical examinations, medical histories, and clinical and laboratory tests), and then eliminates alternative causes based on physical examinations, clinical testing, and the patient's case history.

The recent changes to the Sixth Circuit's application of differential diagnosis can be seen in the cases Cutlip v. Norfolk Southern Corporation, et al., 6th Cir., No. L-02-1051, (4/11/2003) and Best v. Lowe's Home Centers Inc., 6th Cir., No. 08-5924, (4/16/2009).These cases exemplify and discuss how differential diagnosis is no longer being used to determine the specific cause of a medical problem or disease, but is now being used to determine the general cause of these injuries through the addition and elimination of possible causes until the most probable cause is found. Additionally, the Second Circuit case, Ruggiero v. Warner-Lambert Co. (2005), 424 F.3d 249, further demonstrates how the Sixth Circuit's general application of differential diagnosis is much greater than that of other Circuits.

Ruggiero v. Warner-Lambert Co.

In Ruggiero, the Second Circuit severely limited the application of differential diagnosis, holding that it may only be used to establish specific causation. Further, the Second Circuit held that it will deny admissibility of any opinion concerning causation through the use of differential diagnosis unless the opinion is also established independently through Daubert-type standards. In Ruggiero, the plaintiff filed an action alleging her husband suffered from liver failure and death through his use of a drug manufactured by the defendant. In determining the cause of the husband's liver failure, the plaintiff's expert relied only on a review of his medical records. In its opinion, the Ruggiero court held that "differential diagnosis is a patient-specific process of elimination that medical practitioners use to identify the most likely cause of a set of signs and symptoms from a list of possible causes."

Ruggiero's specific causation requirement is significantly more rigid than the Sixth Circuit's recent decisions allowing differential diagnosis to be used to establish general causation. And as exemplified by the Cutlip and Best cases below, the Sixth Circuit's acceptance of differential diagnosis to establish general causation is less likely to prevent a plaintiff from recovery where scientific studies do not exist.

Cutlip v. Norfolk Southern Corporation

In Cutlip v. Norfolk Southern Corporation, et al., supra, the Sixth Circuit found that differential diagnosis can defeat a Daubert challenge (i.e. challenge of an expert's testimony) and that the admission of a differential diagnosis opinion allows an injured party to recover where no scientific research exists. In Cutlip, a plaintiff sued his former employer, a railroad company, alleging that plaintiff contracted asthma as a result of repeated, direct exposures to diesel fumes during his employment. After being thoroughly examined and tested for numerous diseases and disorders, plaintiff's experts, through a process of elimination, attributed plaintiff's asthma to diesel fume exposure during plaintiff's employment at the railroad.

In its holding, the Cutlip court found that differential diagnosis is not only a standard diagnostic tool in medicine, but that a sound differential diagnosis satisfies the Daubert requirements for reliability. The court further held that differential diagnosis precludes the need "for evidence of a dose/response relationship or the threshold phenomenon" and that the admission of differential diagnosis opinions and testimony, in lieu of the dose/relationship or threshold phenomenon, decreases the likelihood that injured parties would be precluded from recovery where no scientific studies exist.

Best v. Lowe's Home Centers Inc.

In Best v. Lowe's Home Centers Inc., supra, the plaintiff appealed the trial court's decision to exclude his medical expert. Here, the plaintiff alleged that he lost his sense of smell after a pool chemical spilled onto his face and clothing during a shopping trip to Lowe's. The Sixth Circuit reversed the court's decision excluding the plaintiff's expert as unreliable for using differential diagnosis to diagnose his medical problem. In doing so, the court of appeals acknowledged the relevance of differential diagnosis and its application in the court system. For the first time, the court provided guidance for trial courts.

In Best, the plaintiff complained that immediately following his exposure to the chemical, he suffered from irritation to his skin, mouth and nasal passages and that shortly thereafter, he lost his sense of smell. The plaintiff's expert completed numerous examinations of the plaintiff, including a thorough medical history and the administering of smell identification tests. Additionally, the medical expert found damaging information concerning the pool chemical which specifically warned about the product's irritation effect on mucous membranes and upper respiratory tract.

The Best court found that differential diagnosis is "an appropriate method for making a determination of causation for an individual instance of a disease" and adopted the differential diagnosis test used by the Third Circuit Court of Appeals. This provides courts in the Sixth Circuit with specific guidelines when seeking to admit causation evidence stemming from differential diagnosis opinions. When meeting these requirements, the Sixth Circuit held that differential diagnosis is an accepted scientific methodology capable of defeating a Daubert challenge.

To be admissible, a physician or expert must:

  1. objectively establish, to the fullest possible extent, the nature of the injury,
  2. "rule in" one or more causes of the injury using accepted methodology, and
  3. engage in standard diagnostic techniques by which doctors normally rule out alternative causes to reach a conclusion as to which cause is most likely.

Additionally, under the third requirement, if the physician engaged in a limited number of accepted diagnostic techniques used by other physicians when ruling out alternative causes, the physician must explain why his conclusions are reliable and why any suggested alternative causes were ruled out.

Finally, the Sixth Circuit's application of differential diagnosis is not a blanket application. Differential diagnosis is still subject to Daubert challenges where the use of multiple unique testing methods may not meet the burden of the differential diagnosis test. Experts with specialized skills (i.e. research scientists and chemists) may be held to higher standards than other experts, which may cause a court to reject differential diagnosis findings.