## Facts

- David L. Best visited a Lowe’s store in Knoxville, Tennessee to buy swimming-pool chemicals and lifted a container of “Aqua EZ Super Clear Clarifier” from a shelf.
- An unknown amount of the chemical splashed onto his face and clothing; the container was allegedly punctured when a Lowe’s employee opened the shipping box.
- Best immediately experienced burning and irritation to skin and airways, dizziness, and shortness of breath, and he sought emergency-room care the same day.
- Months later, Best saw Dr. Francisco Moreno, a board-certified otolaryngologist with chemical-engineering training and experience.
- Moreno took a history, performed an examination, and administered the University of Pennsylvania Smell Identification Test (UPSIT), which was consistent with complete anosmia.
- Moreno reviewed product-safety information describing the chemical as hazardous and irritating to respiratory and mucous membranes, and consulted medical literature regarding potential nasal/olfactory injury from chemical exposure.
- Using differential diagnosis, Moreno considered and ruled out common alternative causes of anosmia (including viral infection, trauma, tumor, surgery, and medications) based on history and clinical information.
- Moreno opined that Best’s exposure to the Aqua EZ chemical was the most likely cause of Best’s anosmia.
- Best sued Lowe’s, alleging the punctured container and exposure caused permanent anosmia.

## Issues

1. Whether the district court abused its discretion under Federal Rule of Evidence 702 and Daubert by excluding Moreno’s specific-causation testimony as unreliable, where it was based on clinical examination, standardized smell testing, safety information, literature, and differential diagnosis.
2. Whether, if the expert exclusion was improper, summary judgment for Lowe’s based on lack of admissible causation proof must be reversed.

## Decision

- The Sixth Circuit held the district court abused its discretion in excluding Moreno’s causation testimony.
- The court concluded Moreno used a generally accepted and reliable differential-diagnosis method and supported it with testing, history, and review of safety information and literature.
- The court rejected the district court’s demand for dose quantification and definitive epidemiological/animal studies as prerequisites to admissibility in an individual toxic-exposure case.
- The court held criticisms of Moreno’s conclusion went to weight and cross-examination, not admissibility.
- Because Moreno’s testimony was admissible and created a genuine dispute on causation, the court reversed summary judgment and remanded.

## Legal Principles

- Trial courts must act as gatekeepers under Rule 702 and Daubert, but exclusion of expert testimony is generally the exception, not the rule.
- A physician’s differential diagnosis can be a reliable basis for specific-causation testimony when the expert identifies plausible causes, reasonably rules out obvious alternatives, and explains the basis for the conclusion.
- An expert need not rule out every possible alternative cause; remaining uncertainty ordinarily affects weight rather than admissibility.
- Lack of precise dose-response data or definitive epidemiological proof does not automatically render a clinically grounded causation opinion inadmissible in an individual exposure case.
- Where expert medical causation testimony is necessary to prove causation, erroneous exclusion may require reversal of summary judgment entered for lack of causation evidence.

## Conclusion

The Sixth Circuit held that an ENT physician’s causation opinion grounded in examination, objective smell testing, safety information, literature review, and differential diagnosis satisfied Rule 702 and Daubert, making exclusion an abuse of discretion; with that evidence, summary judgment for Lowe’s could not stand and the case was remanded.


Best v. Lowe’s Home Ctrs., Inc., 563 F.3d 171 (6th Cir. 2009)