Experts — Differential Diagnosis Is OK under Daubert, But Expert Cannot Neglect to Consider a Potentially Explanatory Hypothesis — Elimination of Potential Causes Must Be Founded on More Than Subjective Belief or Speculation (Good Quote)

Nelson v. Matrixx Initiatives, Inc., 2015 U.S. App. LEXIS 1620 (9th Cir. Feb. 2, 2015):

In a toxic tort case in California, a plaintiff must show both general causation, which is "that the substance at issue was capable of causing the injury alleged," and specific causation, which is "that the substance caused, or was a substantial factor in causing, the specific plaintiff's injury." Avila v. Willits Envtl. Remediation Trust, 633 F.3d 828, 836 (9th Cir. 2011) (citing Jones v. Ortho Pharm. Corp., 209 Cal. Rptr. 456, 462 (Cal. Ct. App. 1985)). General and specific causation "must be proven within a reasonable medical probability based upon competent [*2]  expert testimony." Jones, 209 Cal. Rptr. at 460.

Nelson offered Drs. Davis and Hwang as experts on the issue of specific causation, and both Drs. Davis and Hwang relied on differential diagnosis for their expert opinions that Nelson's smell loss was caused by Zicam. "Differential diagnosis is a common scientific technique, and federal courts, generally speaking, have recognized that a properly conducted differential diagnosis is admissible under Daubert." Clausen v. M/V New Carissa, 339 F.3d 1049, 1057 (9th Cir.  2003). However, "expert testimony that neglects to consider a hypothesis that might explain the clinical findings under consideration may also be unreliable." Id. at 1058. Elimination of the potential causes "must be founded on more than 'subjective beliefs or unsupported speculation.'" Id. (quoting Claar v. Burlington N. R.R. Co., 29 F.3d 499, 502 (9th Cir. 1994)).

The district court excluded Dr. Davis's testimony and expert report, and Dr. Hwang's testimony, because it found that neither expert was able to provide a reliable method for ruling in Zicam, or ruling out age or the cold virus as the cause of Nelson's smell loss. As the district court states, "[b]oth age and the cold virus are well-established, common causes of smell loss." The district court did not abuse its discretion in concluding that Nelson's two expert witnesses [*3]  were not sufficiently reliable on the issue of specific causation, and in excluding both experts on that basis.

After excluding Drs. Davis and Hwang, the court granted Matrixx's motion for summary judgment because Nelson had provided no additional expert testimony on the issue of specific causation and therefore could not prove a necessary element in his case. Without any reliable expert witness testimony on specific causation, the district court correctly granted Matrixx's motion for summary judgment.

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