Sixth Circuit Adopts Differential Diagnosis Admissibility Standard

April 16, 2009

On April 16, 2009, a unanimous Sixth Circuit panel adopted a standard to determine whether causation evidence based upon a differential diagnosis may be admissible under Federal Rule of Evidence 702. In Best v. Lowe’s Home Centers, Inc., 563 F.3d 171 (6th Cir. 2009), the Sixth Circuit held that the District Court improperly excluded an expert’s opinion that plaintiff’s loss of the sense of smell was caused by a pool chemical spill. Best is important not simply because it creates the standard for the admissibility of differential diagnosis in the Sixth Circuit, but because this decision seems to adopt a less stringent standard for the admissibility of causation testimony based upon differential diagnosis.

At trial, Best attempted to introduce expert testimony of a board-certified otolaryngologist to establish a causal link between the chemical spill and his lost sense of smell. Best’s expert had formed his opinion by considering a list of the possible causes of such an injury, including virus, accident, tumor, surgery, exposure to chemicals, and Best’s use of ten different medications. The expert noted that the chemical spill was quite concentrated and relied on Best’s testimony that the chemical splashed on his face and clothing. The expert ruled out nine of plaintiff’s medications as the cause based upon the expert’s personal knowledge of the side effects of those medications. But he did not rule out the last medication and did not rely upon any epidemiological studies to support his opinion. The District Court excluded the expert testimony holding that the differential diagnosis employed by the expert was “unscientific speculation.” The Sixth Circuit disagreed and held that the District Court had improperly excluded the expert’s evidence.

Differential diagnosis is a diagnostic method physicians use to assess what ails a patient. It involves the process of ruling in and ruling out possible diagnoses based on symptoms, test results, and other medical evidence. Often doctors may not have a great deal of evidence as to why a specific cause is on the list of potential causes nor is there a lot of evidence as to why other potential causes are not considered or ruled out.

The Sixth Circuit relied heavily upon In re Paoli Railroad Yard PCB Litigation, 35 F.3rd 717 (3rd Cir. 1994) to create the Best test. According to Best, a medical causation opinion in the form of a doctor’s differential diagnosis is reliable and admissible when: “a doctor ascertains, to the extent possible, the nature of the patient’s injury . . . ‘rules in’ one or more causes of the injury using a valid methodology, and engages in ‘standard diagnostic techniques by which doctors normally rule out alternative causes’ to reach a conclusion as to which cause is most likely.”

The stated purpose of this test is to require the same level of scientific rigor in litigation driven testimony as would be found in the treatment of patients. But this test may not satisfy that goal because language in the opinion indicates that this is a less stringent standard than would be applied to “real” scientists. The Sixth Circuit softened this standard by stating: “Dr. Moreno’s diagnosis might not stand up to the exacting scrutiny if he were testifying as a research scientist or a chemist, but he is neither of those.” Best, 563 F.3d at 181-182. The opinion arguably further softened the prior standard by noting, in dicta, that plaintiff’s expert testimony may not have even been necessary to defeat summary judgment because the symptoms occurred so quickly after the alleged exposure to the pool chemical. These two statements undermine the Sixth Circuit’s stated goal of providing a manner by which to distinguish reliable from unreliable differential diagnosis and open the door for plaintiff’s attorneys to introduce causation evidence, that could not meet the standard to which research scientists are held, under the cloak of differential diagnosis. It remains to be seen how Best will be interpreted and applied in the Sixth Circuit, but for now the clear import of Best is that causation evidence derived from a differential diagnosis may not be scientifically scrutinized as intensely as other causation evidence, such as of research scientists or chemists.