Sixth Circuit Formulates “Differential Diagnosis” Standard For Medical-Causation

In a negligence action following accidental chemical spill on a store customer, the Sixth Circuit concluded the trial court erred by failing to consider whether a medical-causation expert could use “differential diagnoses”; the circuit formulated a differential diagnosis standard and remanded the case after concluding expert testimony was admissible under the new differential diagnosis standard, in Best v. Lowe's Home Centers, Inc., 563 F.3d 171 (6th Cir. April 16, 2009) (No. 08-5924)

In medical causation cases, one form of expert testimony may involve “differential diagnosis.” Under this approach, “a physician determines what disease process caused a patient’s symptoms. The physician considers all relevant potential causes of the symptoms and then eliminates alternative causes based on a physical examination, clinical tests, and a thorough case history.” Hardyman v. Norfolk & W. Ry. Co., 243 F.3d 255, 260 (6th Cir. 2001) (quoting Federal Judicial Center, Reference Manual on Scientific Evidence 214 (1994)). The Sixth Circuit had not previously articulated the standard to apply a “differential diagnosis” for expert medical causation testimony.

In the case, plaintiff Best visited a Lowe’s Home Center to purchase swimming pool chemicals. When he raised a plastic container of Aqua EZ, product spilled onto his face and clothing. Apparently, the container had been accidentally punctured by a knife used to open a box before stacking the containers on the store shelf. He was treated in an emergency room for the spill. Later he received treatment from Dr. Moreno, a board-certified otolaryngologist and a former chemical engineer, who concluded that permanent loss of smell (anosmia) resulted from the spill. Under the differential diagnosis, the expert identified possible causes for loss of smell (such as “a virus, an accident, tumors to the brain, surgery into the brain, or exposure to chemicals,” medications, or other unknown reasons) and excluded them. Best, 563 F.3d at 174.

Best filed an action in state court which was removed to federal court. After Dr. Moreno’s deposition, the defense moved to exclude the medical expert testimony. The court granted the motion after deciding the expert testimony was based on “unscientific speculation” and several “red flags” based on another district court opinion. See Best v. Lowe’s Home Centers, Inc., No. 3:04-CV-294, 2008 WL 2359986 at *9 (E.D. Tenn. June 5, 2008). Without any other proof of causation, the district court granted summary judgment for Lowe’s on the anosmia claim. Other claims were withdrawn. On appeal, Best challenged the exclusion of the medical expert testimony.

The Sixth Circuit reversed. Because the trial court failed to “recognize that differential diagnosis is a valid technique that often underlies reliable medical-causation testimony,” the circuit did not review the trial court ruling under the deferential abuse of discretion standard but under a de novo standard. Best, 563 F.3d at 178 (citing United States v. 2903 Bent Oak Highway, 204 F.3d 658, 665 (6th Cir. 2000) (the circuit “extends a high degree of deference to the district court’s decision” under the abuse-of-discretion standard “only if the district court properly understood the pertinent law”))] The circuit noted that it had not provided standards for providing differential diagnoses: “The problem is that no case in this circuit has previously provided detailed guidance for the district courts in separating reliable differential diagnoses from unreliable ones.” Best, 563 F.3d at 179. Drawing upon the Third Circuit opinion in In re Paoli Railroad Yard PCB Litigation, 35 F.3d 717 (3d Cir. 1994), the Sixth Circuit took the opportunity to formulate a “differential-diagnosis test”:

“A medical-causation opinion in the form of a doctor’s differential diagnosis is reliable and admissible where the doctor (1) objectively ascertains, to the extent possible, the nature of the patient’s injury, see [In re Paoli Railroad Yard PCB Litigation, 35 F.3d] at 762 (“A physician who evaluates a patient in preparation for litigation should seek more than a patient’s self-report of symptoms or illness and . . . should . . . determine that a patient is ill and what illness the patient has contracted.”), (2) ‘rules in’ one or more causes of the injury using a valid methodology, and (3) engages in ‘standard diagnostic techniques by which doctors normally rule out alternative causes’ to reach a conclusion as to which cause is most likely. Id. at 760. In connection with the third “rules out” prong, if the doctor “engage[s] in very few standard diagnostic techniques by which doctors normally rule out alternative causes,” the doctor must offer a “good explanation as to why his or her conclusion remain[s] reliable.” Id. Similarly, the doctor must provide a reasonable explanation as to why “he or she has concluded that [any alternative cause suggested by the defense] was not the sole cause.” Id. at 758 n.27.”
Best, 563 F.3d at 179.


In applying the new formulation, the circuit concluded the expert opinion satisfied “the minimum threshold for admissibility.” Best, 563 F.3d at 180. First, Dr. Moreno used “a well-recognized test -- the UPSIT -- to confirm Best’s complaint that he could not smell.” Second, “Dr. Moreno compiled a list of possible causes for the injury, including virus, accident, brain tumor, brain surgery, exposure to chemicals, medications, or an ideopathic (unknown) cause.” Third, the doctor ruled out alternate causes, including “nine of Best’s ten medications as potential causes of anosmia.” Best, 563 F.3d at 181.

Finally, the circuit distinguished between weight of the evidence factors from admissibility. For example, the circuit noted that challenges to the methodology went to the weight of the evidence and not admissibility. Best, 563 F.3d at 182 (citing Kudabeck v. Kroger Co., 338 F.3d 856, 861-62 (8th Cir. 2003) (“[A]ttacks regarding the completeness of [a doctor’s] methodology go to the weight and not the admissibility of his testimony.”); Heller v. Shaw Indus., Inc., 167 F.3d 146, 157 (3d Cir. 1999) (describing a case in which the “district court erred in excluding expert medical testimony because a defendant’s suggested alternative causes (once adequately addressed by plaintiff’s expert) affect the weight that the jury should give the expert’s testimony and not the admissibility of that testimony”))] As the circuit summarized:

“Lowe’s has pointed to several potential problems with Dr. Moreno’s expert opinion. But our function is not to determine whether the opinion is airtight and conclusively proves the cause of Best’s anosmia. Rather, the court’s role as gatekeeper is to decide whether Dr. Moreno performed his duties as a diagnosing physician to the professional level expected in his field. See Kumho Tire Co. v. Carmichael, 526 U.S. 137, 152 (1999). In light of this standard, we conclude that Dr. Moreno’s differential diagnosis testimony meets the threshold level of admissibility under Daubert.”
Best, 563 F.3d at 183-84.


The Sixth Circuit noted that “[a]n ‘overwhelming majority of the courts of appeals’ agree, and have held ‘that a medical opinion on causation based upon a reliable differential diagnosis is sufficiently valid to satisfy the first prong [reliability] of the Rule 702 inquiry.’” Best, 563 F.3d at 178 (quoting Westberry v. Gislaved Gummi AB, 178 F.3d 257, 263 (4th Cir. 1999) (collecting cases from the First, Second, Third, Ninth, and D.C. Circuits)). The Best opinion is the latest to specify the standards that should apply under this form of expert analysis.

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