                         RECOMMENDED FOR FULL-TEXT PUBLICATION
                              Pursuant to Sixth Circuit Rule 206
                                      File Name: 09a0148p.06

                 UNITED STATES COURT OF APPEALS
                                  FOR THE SIXTH CIRCUIT
                                    _________________


                                                 X
                                                  -
 DAVID L. BEST,
                                                  -
                                 Plaintiff-Appellant,
                                                  -
                                                  -
                                                      No. 08-5924
          v.
                                                  ,
                                                   >
                                                  -
                         Defendant-Appellee. -
 LOWE’S HOME CENTERS, INC.,
                                                  -
                                                 N
                   Appeal from the United States District Court
                for the Eastern District of Tennessee at Knoxville.
             No. 04-00294—C. Clifford Shirley, Jr., Magistrate Judge.
                                    Argued: March 12, 2009
                              Decided and Filed: April 16, 2009
                                                                                               *
      Before: MARTIN and GILMAN, Circuit Judges; ZOUHARY, District Judge.

                                      _________________

                                           COUNSEL
ARGUED: Robert E. Pryor, Jr., PRYOR, FLYNN, PRIEST & HARBER, Knoxville,
Tennessee, for Appellant. Clinton J. Woodfin, SPICER, FLYNN & RUDSTROM,
Knoxville, Tennessee, for Appellee. ON BRIEF: Robert E. Pryor, Jr., PRYOR,
FLYNN, PRIEST & HARBER, Knoxville, Tennessee, for Appellant. Clinton J.
Woodfin, SPICER, FLYNN & RUDSTROM, Knoxville, Tennessee, for Appellee.
                                      _________________

                                            OPINION
                                      _________________

        RONALD LEE GILMAN, Circuit Judge. David L. Best claims to suffer from
permanent anosmia—the loss of his sense of smell—as a result of a pool chemical
spilling onto his face and clothing at a Lowe’s Home Center store. After filing suit


        *
           The Honorable Jack Zouhary, United States District Judge for the Northern District of Ohio,
sitting by designation.


                                                  1
No. 08-5924         Best v. Lowe’s Home Centers, Inc.                                Page 2


against Lowe’s, Best planned to introduce the expert testimony of Dr. Francisco Moreno,
a board-certified otolaryngologist (an ear, nose, and throat doctor) and a former chemical
engineer, in order to establish the causal link between the chemical spill and his injuries.
The district court excluded Dr. Moreno’s testimony, holding that the method employed
by the doctor in drawing his conclusions regarding causation was “unscientific
speculation.” This resulted in summary judgment being granted in favor of Lowe’s. For
the reasons set forth below, we REVERSE the judgment of the district court and
REMAND the case for further proceedings consistent with this opinion.

                                 I.   BACKGROUND

A.      Factual background

        Best visited a Lowe’s store in Knoxville, Tennessee in June 2003. Intending to
purchase chemicals for his swimming pool, he located a product called Aqua EZ Super
Clear Clarifier (Aqua EZ). When Best lifted the plastic container from the shelf, an
unknown quantity of the contents splashed onto his face and clothing. The container had
allegedly been accidentally punctured with a knife by the Lowe’s employee who had
opened the shipping box. Best went to the emergency room of a hospital for treatment
on the day that the spill occurred. Four months later, Best sought care and treatment
from Dr. Moreno for the injuries associated with the incident. Dr. Moreno has practiced
medicine as an otolaryngologist since 1982. Before attending medical school, Dr.
Moreno earned a Bachelor of Science degree in chemical engineering. He was
employed as a chemical engineer from 1968 until 1972.

        At the time of his initial visit to Dr. Moreno, Best described the incident at
Lowe’s. He said that the spilled product had a strong odor, and that immediately
thereafter he had suffered from irritation and burning of his skin, irritation to his nasal
passages and mouth, dizziness, and shortness of breath.        Best also reported that he
experienced clear drainage from his nose following the spill and that he eventually lost
his sense of smell completely. Dr. Moreno was unable to inspect Best’s mucous
No. 08-5924         Best v. Lowe’s Home Centers, Inc.                                Page 3


membranes for physical damage because they are located too far inside the nasal
passages to permit visual examination.

        Best was seen for a second time by Dr. Moreno in January 2007. Dr. Moreno
took a new medical history and again performed a physical examination to the extent
possible in light of the position of the mucous membranes in the nose. At that time, Best
was experiencing rhinitis—otherwise known as a runny or stuffy nose—with swelling
and decreased airflow. Best reported that, during the three-and-a-half year period since
the spill incident, he had struggled with rhinitis, anosmia, and dizzy spells.

        In April 2008, Dr. Moreno administered to Best the University of Pennsylvania
Smell Identification Test (UPSIT), a standardized test of olfactory function. The test
involves various sample chemicals, requiring the test subject to choose one of four
descriptions of each sample’s scent. Best scored a six on the test, a score consistent with
complete anosmia.

        Dr. Moreno testified in his deposition that “[l]oss of smell is caused by either a
virus, an accident, tumors to the brain, surgery into the brain, or exposure to chemicals.”
He also conceded that sometimes anosmia is idiopathic, meaning that it occurs for
unknown reasons, and that some medications can cause a loss of the sense of smell. Dr.
Moreno proceeded to list the following medications that Best reported taking at the time
of his chemical exposure: aspirin, Atenolol, Effexor, hydrochlorithiazide, Lescol,
Letensin, moxamorphin, OxyContin, Protonix, and Remeron. Dr. Moreno stated that
Atenolol and Lotensin are for blood pressure; aspirin, moxamorphin, and OxyContin are
for pain; Effexor is for depression; hydrochlorothiazide is a fluid pill; and Protonex is
for the stomach. He was unfamiliar with the drug Lescol. Referring to all of the
medications, he stated that “[i]n my practice, with the patients that I have seen . . . over
the years . . . , I have never seen an anosmia caused from the use of these medications.”
He also said that he had looked up all of the medications except Lescol in the course of
his practice. Dr. Moreno was unable to list the general types of medications that can
cause a loss of the sense of smell.
No. 08-5924        Best v. Lowe’s Home Centers, Inc.                                Page 4


       Lowe’s provided Best’s attorney with a one-page document identifying the pool
chemical as Aqua EZ. The document describes the product as a “thick blue liquid”
containing cationic polymers that attract foreign particles in the pool water so that they
can be more efficiently removed by the filtration system. After receiving this document,
Best’s attorney obtained a Material Safety Data Sheet (MSDS) prepared by Ciba
Specialty Chemicals Corporation, the supplier of the active ingredient in Aqua EZ. Dr.
Moreno reviewed the MSDS, which describes the characteristics of the active ingredient.

       The relevant ingredient is an organic cationic polyelectrolyte. Specifically, the
compound is a homopolymer with the name 2-Propen-1-aminium, N, N-dimethyl-N-2-
propenyl-chloride. The MSDS identifies the chemical as “hazardous” and states that
“[p]rolonged or repeated contact may cause eye and skin irritation.” Primary routes of
entry for the compound are listed as “Ingestion, Skin, Inhalation, Eyes.” According to
the MSDS, if the chemical is inhaled, the person should be “[r]emove[d] to fresh air, if
not breathing give artificial respiration. If breathing is difficult, give oxygen and get
immediate medical attention.” The Handling Instructions state: “Do not inhale . . . . Use
only with adequate ventilation.” Under the heading “Engineering Controls,” the MSDS
instructs: “Work in well ventilated areas. Do not breathe vapors or mist.” The MSDS
also notes that “Acute Inhalation Toxicity” for the compound has not been determined.
Dr. Moreno later reviewed a second MSDS, published by Sigma-Aldrich, another
supplier of the relevant compound. That MSDS confirmed that the compound is
“irritating to the mucous membrane and upper respiratory tract” and that it “[m]ay be
harmful if inhaled.”

       Dr. Moreno concluded, based on the MSDS information, that the inhalation of
Aqua EZ has the potential to cause damage to the nasal and sinus mucosa and the nerve
endings of the olfactory bulb. According to Dr. Moreno, the culprit components of the
polymer in question include a chlorine derivative and an ammonium derivative. He
offered his opinion that “a chemical burn can cause a loss of smell on a time basis” due
to “scarring of the tissue,” and reported that he has treated other chemical exposures with
anosmic side effects following exposure to chlorine derivatives. But Dr. Moreno did not
No. 08-5924        Best v. Lowe’s Home Centers, Inc.                                   Page 5


know the precise amount of the offending chemical that Best had been exposed to, nor
was he able to determine the threshold level of exposure that could cause harm. Dr.
Moreno summarized his diagnosis regarding causation this way:

       The patient had an accident, chemical was spilled, the patient cannot
       smell. If we have any trust in the patient at all, all I can say is he cannot
       smell. I did test him, his test was positive in the fact that he was
       anosmic. All I can tell you is that exposure to the—the only exposure
       that he had at the time that I talked to him was exposure to this chemical.
       There was nothing else in his history that dictated the fact that he was
       anosmic otherwise.
In short, because of the temporal relationship between Best’s exposure to the chemical
and the onset of his symptoms, in conjunction with a principled effort to eliminate other
possible causes of anosmia, Dr. Moreno formed the opinion that the inhalation of Aqua
EZ caused Best to lose his sense of smell.

B.     Procedural history

       Best originally filed suit against Lowe’s in a Tennessee trial court. Lowe’s
timely removed the case to the federal district court based upon diversity of citizenship
under 28 U.S.C. § 1332. After Best identified Dr. Moreno as an expert witness, counsel
for Lowe’s took the doctor’s deposition. Lowe’s then moved for the exclusion of Dr.
Moreno’s testimony regarding the cause of Best’s injury and also moved for summary
judgment.     The district court excluded Dr. Moreno’s proposed testimony after
concluding that the doctor’s opinion was too speculative. Best v. Lowe’s Home Centers,
Inc., No. 3:04-CV-294, 2008 WL 2359986 at *9 (E.D. Tenn. June 5, 2008). Because
Best presented no other evidence to carry his burden of proof on the element of
causation, the district court granted partial summary judgment in favor of Lowe’s on
Best’s anosmia claim. Id. Best later withdrew his claims for the less serious injuries and
losses that he allegedly suffered as a result of the chemical spill. The court accordingly
granted summary judgment in full to Lowe’s. Best now appeals the district court’s
decision to exclude Dr. Moreno’s testimony.
No. 08-5924         Best v. Lowe’s Home Centers, Inc.                                    Page 6


                                     II. ANALYSIS

A.      Standard of review

        We apply the abuse-of-discretion standard in reviewing a district court’s decision
regarding the admissibility of expert testimony. Hardyman v. Norfolk & W. Ry. Co., 243
F.3d 255, 258 (6th Cir. 2001). “A district court abuses its discretion if it bases its ruling
on an erroneous view of the law or a clearly erroneous assessment of the evidence.”
Brown v. Raymond Corp., 432 F.3d 640, 647 (6th Cir. 2005) (internal quotation marks
omitted).

B.      Admissibility of expert testimony on medical causation

        Rule 702 of the Federal Rules of Evidence controls the admissibility of all types
of expert testimony. The rule provides as follows:

        If scientific, technical, or other specialized knowledge will assist the trier
        of fact to understand the evidence or to determine a fact in issue, a
        witness qualified as an expert by knowledge, skill, experience, training,
        or education, may testify thereto in the form of an opinion or otherwise,
        if (1) the testimony is based upon sufficient facts or data, (2) the
        testimony is the product of reliable principles and methods, and (3) the
        witness has applied the principles and methods reliably to the facts of the
        case.
According to Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), a
district court’s task in assessing evidence proffered under Rule 702 is to determine
whether the evidence “both rests on a reliable foundation and is relevant to the task at
hand.” Id. at 597. The district court must consider “whether the reasoning or
methodology underlying the testimony is scientifically valid.” Id. at 592-93.

        Daubert attempts to strike a balance between a liberal admissibility standard for
relevant evidence on the one hand and the need to exclude misleading “junk science” on
the other. See Amorgianos v. Nat’l R.R. Passenger Corp., 303 F.3d 256, 267 (2d Cir.
2002). There is no “definitive checklist or test” for striking this balance, but the
Supreme Court in Daubert set forth a number of factors that typically “bear on the
inquiry.” 509 U.S. at 593. These include whether the theory or technique in question
No. 08-5924         Best v. Lowe’s Home Centers, Inc.                                Page 7


“can be (and has been) tested,” whether it “has been subjected to peer review and
publication,” whether it has a “known or potential rate of error,” and finally, whether the
theory or technique enjoys general acceptance in the relevant scientific community. Id.
at 594. The Rule 702 inquiry is “a flexible one,” and “[t]he focus . . . must be solely on
principles and methodology, not on the conclusions they generate.” Id. at 594-95. An
expert who presents testimony must “employ[] in the courtroom the same level of
intellectual rigor that characterizes the practice of an expert in the relevant field.”
Kumho Tire Co. v. Carmichael, 526 U.S. 137, 152 (1999).

C.      The district court’s analysis

        In applying Daubert to Dr. Moreno’s opinion, the district court focused on the
analysis found in the case of Downs v. Perstorp Components, Inc., 126 F. Supp. 2d 1090
(E.D. Tenn. 1999). The district court in Downs excluded the testimony of a physician
who concluded that an incident involving skin contact and inhalation exposure to a
polyurethane polymer caused the plaintiff to suffer “severe facial pain, sensory
abnormalities, visual field losses, impaired balance, slowed reaction time, and recall-
memory impairment.” Id. at 1093. That opinion in turn relied heavily on Moore v.
Ashland Chemical Inc., 151 F.3d 269 (5th Cir. 1997) (en banc) (affirming a district
court’s exclusion of a physician’s medical-causation opinion). Downs, 126 F. Supp. 2d
at 1120-22. Downs also identified and considered a list of “red flags” provided by a
treatise on evidence, and the district court in the present case followed suit. Id. at 1125-
28 (citing 2 Saltzburg, Martin & Kapra, Federal Rules of Evidence Manual, 1229-37 (7th
ed. 1998)).

        The “red flags” that were employed by both the Downs court and the district
court in the present case to purportedly demonstrate a lack of reliability were the
following: (1) improper extrapolation, (2) reliance on anecdotal evidence, (3) reliance
on temporal proximity, (4) insufficient information about the case, (5) failure to consider
other possible causes, (6) lack of testing, and (7) subjectivity. Downs, 126 F. Supp. 2d
at 1125-28; Best, 2008 WL 2359986 at *5-7. According to the district court below, Dr.
No. 08-5924        Best v. Lowe’s Home Centers, Inc.                                Page 8


Moreno’s opinion regarding the cause of Best’s alleged loss of his sense of smell raised
every red flag except number 6 (lack of testing). Id. at *8-9.

       Specific faults perceived by the district court included Dr. Moreno’s
extrapolation from the chemical’s ability to irritate mucous membranes to a conclusion
that such irritation could lead to scarring and anosmia. Id. at *8. The court also faulted
Dr. Moreno for relying on past experiences with patients who had been exposed to
different chlorine derivatives and the “temporal proximity between the incident and the
reported injury.” Id. It next criticized Dr. Moreno for having “insufficient information
about the case” because he “reli[ed] on [Best]’s general and subjective report of the
accident, an examination of [Best], and the MSDS sheet.” Id. The court then noted that
Dr. Moreno was unfamiliar with Lescol, one of Best’s medications, and concluded that
his opinion was faulty for “failure to consider other possible causes.” Id. In light of Dr.
Moreno’s testimony that testing the effects of hazardous chemicals on humans is not
appropriate, the district court decided not to hold the lack of such testing against him.
Id. Having concluded that the six other red flags were raised, however, the court held
that Dr. Moreno’s opinion was inadmissible “unscientific speculation.” Id.

D.     Differential diagnosis

       Dr. Moreno employed a methodology known as “differential diagnosis” in
forming his opinion. Differential diagnosis is “[t]he method by which 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)).

       As described above, Dr. Moreno formed his opinion regarding Best’s alleged loss
of his sense of smell by considering a list of the possible causes of such an injury—“a
virus, an accident, tumors to the brain, surgery into the brain, or exposure to
chemicals”—as well as Best’s use of medications and the possibility of another,
unknown (idiopathic) cause. He took note of the temporal proximity between Best’s
No. 08-5924        Best v. Lowe’s Home Centers, Inc.                                Page 9


exposure to Aqua EZ and discovered that the pool clarifier contained a chemical that,
according to the MSDS, is “irritating to the mucous membrane and upper respiratory
tract” and “[m]ay be harmful if inhaled.” Although Dr. Moreno was unable to quantify
the level of Best’s exposure, he noted that the chemical was quite concentrated—a “little
bottle” is poured into a “whole swimming pool”—and relied on Best’s report that the
material splashed onto his face and clothing. Dr. Moreno ruled out medications as the
cause, based on his knowledge of the side effects of nine out of Best’s ten medications
(he had no information about the tenth), and also because he had never known of a
patient who had used any of the medications and developed anosmia. Finally, Dr.
Moreno ruled out idiopathic anosmia because of the remote likelihood that some
unknown cause would bring about anosmia “all of a sudden” around the same time as
an exposure to a chemical that is known to irritate the nasal mucous membranes.

       This court recognizes differential diagnosis as “an appropriate method for
making a determination of causation for an individual instance of disease.” Hardyman,
243 F.3d at 260. An “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.”
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). Differential diagnosis is
considered to be “a standard scientific technique of identifying the cause of a medical
problem by eliminating the likely causes until the most probable one is isolated.”
Hardyman, 243 F.3d at 260 (quoting Westberry, 178 F.3d at 262).

       The district court below did not cite Hardyman or any other cases that discuss
the admissibility of causation opinions based on the method of differential diagnosis.
Because the court did not recognize that differential diagnosis is a valid technique that
often underlies reliable medical-causation testimony, its conclusions are not entitled to
the deference that they would otherwise receive under the abuse-of-discretion standard
of review. See United States v. 2903 Bent Oak Highway, 204 F.3d 658, 665 (6th Cir.
2000) (explaining that we will “extend[] a high degree of deference to the district court’s
No. 08-5924         Best v. Lowe’s Home Centers, Inc.                               Page 10


decision” under the abuse-of-discretion standard “only if the district court properly
understood the pertinent law”). We will accordingly analyze Dr. Moreno’s differential-
diagnosis method de novo to determine whether his methodology led to a reliable,
admissible opinion under Rule 702.

        Not every opinion that is reached via a differential-diagnosis method will meet
the standard of reliability required by Daubert. See, e.g., Rolen v. Hansen Beverage Co.,
193 F. App’x 468, 474 n.4 (6th Cir. 2006) (holding that a doctor’s opinion did not meet
“the kind of methodological rigor that characterizes acceptable differential diagnosis”).
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. We
find the Third Circuit’s opinion in the case of In re Paoli Railroad Yard PCB Litigation,
35 F.3d 717 (3d Cir. 1994), instructive in this regard.

        In Paoli Railroad Yard, the court evaluated the differential-diagnosis-based
causation testimony of two physicians regarding the various ailments of a large number
of plaintiffs who lived near a facility where polychlorinated biphenyls (PCBs) were used
for an extended period of time. Id. at 732. The Third Circuit noted that “differential
diagnosis generally is a technique that has widespread acceptance in the medical
community, has been subject to peer review, and does not frequently lead to incorrect
results.” Id. at 758. It also emphasized the individual nature of each differential
diagnosis. Id. (“[T]he steps a doctor has to take to make [a] (differential) diagnosis
reliable are likely to vary from case to case.”). As a result, the court stated that, “to the
extent that a doctor utilizes standard diagnostic techniques in gathering . . . information,”
a finding that “the doctor’s methodology is reliable” is “more likely.” Id. Another
observation by the court was that “performance of physical examinations, taking of
medical histories, and employment of reliable laboratory tests all provide significant
evidence of a reliable differential diagnosis,” and that “their absence makes it much less
likely that a differential diagnosis is reliable.” Id. “The core of differential diagnosis
is a requirement that experts at least consider alternative causes.” Id. at 759.
No. 08-5924         Best v. Lowe’s Home Centers, Inc.                               Page 11


        We hereby adopt the following differential-diagnosis test, adapted from the Third
Circuit’s well-reasoned opinion: 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 id. 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.

        Our approach is similar to those employed in other circuits that recognize
differential diagnosis as a valid basis for medical-causation opinions. See, e.g., Ruggiero
v. Warner-Lambert Co., 424 F.3d 249, 254 (2d Cir. 2005) (“Where an expert employs
differential diagnosis to ‘rule out’ other potential causes for the injury at issue, he must
also ‘rule in’ the suspected cause, and do so using scientifically valid methodology.”
(internal quotation marks omitted)); Glastetter v. Novartis Pharm. Corp., 252 F.3d 986,
989 (8th Cir. 2001) (“In performing a differential diagnosis, a physician begins by
‘ruling in’ all scientifically plausible causes of the plaintiff’s injury. The physician then
‘rules out’ the least plausible causes of injury until the most likely cause remains.”).
No. 08-5924        Best v. Lowe’s Home Centers, Inc.                             Page 12


E.     Dr. Moreno’s opinion was sufficiently reliable to warrant admissibility

       Applying our newly formulated test to Dr. Moreno’s opinion, we conclude that
his methodology meets the minimum threshold for admissibility. We consider each
element of the differential-diagnosis test in turn.

       1.      Dr. Moreno ascertained, to the extent possible, that Best is anosmic

       Dr. Moreno employed a well-recognized test—the UPSIT—to confirm Best’s
complaint that he could not smell. Based on the research that Dr. Moreno had done
regarding tests for anosmia, he concluded that the UPSIT is “as objective as you’re ever
going to get.” Lowe’s has made no attempt to discredit that test.

       Instead, Lowe’s argues that Dr. Moreno had never before administered the
UPSIT and that Best’s score was only one point outside the range for malingering,
suggesting that Best purposely manipulated the result. Lowe’s also points out that when
Best took the UPSIT, Dr. Moreno possessed conflicting information about Best’s
smoking habits—a factor that the doctor knew could affect the test results. In addition,
Lowe’s complains that Dr. Moreno did not examine the record of Best’s visit to the
hospital’s emergency room, during which Best allegedly stated that he did not inhale the
Aqua EZ. But Best also reported at that time that the chemical spilled on his face and
clothing, and he described its strong odor. Dr. Moreno accordingly observed that he
“would have to assume that regardless of the statements made that [Best] had to have
had some exposure.”

       All of Lowe’s attacks on Dr. Moreno’s efforts to ascertain whether Best is
anosmic amount to factual disputes suitable for cross-examination. See Daubert, 509
U.S. at 596 (“Vigorous cross-examination, presentation of contrary evidence, and careful
instruction on the burden of proof are the traditional and appropriate means of attacking
shaky but admissible evidence.”). Where, as here, a doctor has used a reliable method
to conclude that the plaintiff has suffered an injury, potential problems such as those
pointed out by Lowe’s do not warrant the total exclusion of plainly relevant testimony.
No. 08-5924        Best v. Lowe’s Home Centers, Inc.                              Page 13


       2.      Dr. Moreno employed a valid methodology to “rule in”
               Aqua EZ as a potential cause
       Based on his medical knowledge, 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. Lowe’s strongest argument
is that no published material confirms that inhalation of the chemical in Aqua EZ can
cause anosmia. But “there is no requirement that a medical expert must always cite
published studies on general causation in order to reliably conclude that a particular
object caused a particular illness.” Kudabeck v. Kroger Co., 338 F.3d 856, 862 (8th Cir.
2003) (internal quotation marks omitted). Dr. Moreno did not arbitrarily “rule in” Aqua
EZ as a potential cause, but instead concluded from the MSDS sheet and his own
knowledge of medicine and chemistry that the chemical it contains can cause damage
to the nasal and sinus mucosa upon inhalation.

       In addition, Dr. Moreno has treated other patients who developed anosmic
symptoms after inhaling chlorine derivatives. The opinion presented by Dr. Moreno thus
differs markedly from those in cases like Ruggiero v. Warner-Lambert Co., 424 F.3d
249, 254 (2d Cir. 2005), where the Second Circuit excluded a doctor’s opinion that a
particular diabetes medication could cause liver cirrhosis and death because the expert
could not point to anything suggesting such a possibility. Id. at 251-52.

       Another Second Circuit case, McCullock v. H.B. Fuller Co., 61 F.3d 1038 (2d
Cir. 1995), comes much closer to the circumstances in the present case. In McCullock,
the plaintiff developed throat polyps after being exposed to hot-glue fumes. Id. at 1040-
41. Her treating physician, “an experienced medical doctor . . . certified by the
American Board of Otolaryngology,” opined that the plaintiff’s polyps resulted from
“inhalation of the fumes from the hot-glue pot.” Id. at 1042-43. The MSDS sheet for
the hot glue at issue in McCullock contained similar warnings to those in this case,
including: “Avoid breathing vapors/fumes,” and “Vapors and fumes may cause irritation
of the nose, throat and respiratory tract.” Id. at 1040. Despite the defendant’s insistence
that the expert “could not point to a single piece of medical literature that says glue
fumes cause throat polyps,” id. at 1043, the court admitted the doctor’s testimony, citing
No. 08-5924         Best v. Lowe’s Home Centers, Inc.                              Page 14


in support the doctor’s “review of [the] MSDS” and his “training and experience,”
among other things. Id. at 1044. Dr. Moreno’s testimony should likewise be admitted
here.

        3.      Dr. Moreno engaged in standard techniques to “rule
                out” alternate causes
        Having no evidence that virus, accident, brain tumor, or brain surgery were
applicable in Best’s case, Dr. Moreno focused on chemicals, medications, or ideopathic
causes. Dr. Moreno concluded, based on his own experience, that an ideopathic anosmia
would not appear over such a short period of time. He also eliminated nine of Best’s ten
medications as potential causes of anosmia.

        Lowe’s makes much of Dr. Moreno’s failure to eliminate Lescol as a possible
cause. But doctors need not rule out every conceivable cause in order for their
differential-diagnosis-based opinions to be admissible. E.g., Westberry v. Gislaved
Gummi AB, 178 F.3d 257, 266 (4th Cir. 1999) (citing In re Paoli R.R. Yard PCB Litig.,
35 F.3d 717, 764-65 (3d Cir. 1994)). Lowe’s presented no evidence that Lescol might
cause anosmia. If such evidence exists, or if Dr. Moreno failed to consider some other
likely cause, Lowe’s is free to attack Dr. Moreno’s opinion on that basis at trial.

        Admissibility under Rule 702 does not require perfect methodology. Rather, the
expert must “employ[] in the courtroom the same level of intellectual rigor that
characterizes the practice of an expert in the relevant field.” Kumho Tire Co. v.
Carmichael, 526 U.S. 137, 152 (1999). Dr. Moreno’s diagnosis might not stand up to
exacting scrutiny if he were testifying as a research scientist or a chemist, but he is
neither of those. He performed as a competent, intellectually rigorous treating physician
in identifying the most likely cause of Best’s injury.           Any weaknesses in his
methodology will affect the weight that his opinion is given at trial, but not its threshold
admissibility. See, e.g., 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
No. 08-5924        Best v. Lowe’s Home Centers, Inc.                             Page 15


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”).

F.     Dr. Moreno’s opinion is distinguishable from differential-diagnosis
       opinions that have been excluded in other cases
       A review of several cases in which differential-diagnosis testimony has been
excluded further solidifies our conclusion that Dr. Moreno’s opinion falls on the
admissible side of the elusive line separating reliable opinions from “junk science.” In
particular, we believe that the case of Downs v. Perstorp Components, Inc., 126 F. Supp.
2d 1090 (E.D. Tenn. 1999), upon which the district court heavily relied, is materially
distinguishable. For starters, the court in Downs gave great weight to Moore v. Ashland
Chemical Inc., 151 F.3d 269 (5th Cir. 1997) (en banc), a case that takes a minority
position by implicitly rejecting the validity of differential diagnosis in the formulation
of medical-causation opinions. See Westberry, 178 F.3d at 263 (contrasting the
exclusion of a causation opinion in Moore with opinions from several other circuits that
recognize differential diagnosis as a reliable methodology). Reliance on Moore by the
district court in Downs was thus misplaced in light of Glaser v. Thompson Medical Co.,
32 F.3d 969, 977 (6th Cir. 1994), which explicitly recognized the admissibility of
properly developed differential-diagnosis opinions in this circuit.

       Moreover, under a proper differential-diagnosis analysis, the opinion offered by
the expert in Downs differs from Dr. Moreno’s opinion in several key ways. In Downs,
Dr. Kilburn was the plaintiff’s expert who concluded that the plaintiff had suffered
severe brain damage as a result of a single exposure to a polyurethane polymer. 126 F.
Supp. 2d at 1093. The defense presented voluminous testimony from other experts that
Dr. Kilburn’s methods of testing the plaintiff for injury were novel and not generally
accepted in the neuropsychological community. Id. at 1108-15. These competing
experts concluded, based on their own tests, that the plaintiff had no neurological
impairment that could have resulted from the chemical exposure. Id.
No. 08-5924         Best v. Lowe’s Home Centers, Inc.                              Page 16


        Dr. Kilburn therefore did not follow a reliable method to ascertain the nature of
the plaintiff’s injury, and he had no reliable information with which to “rule in” chemical
exposure as a potential cause. He was not able to say which component of the chemical
product might have caused the injury, or point to any objective source suggesting that
such an injury might result from exposure to any component of the chemical mixture at
issue. Id. at 1098. Dr. Kilburn had access to MSDS sheets describing the components
of the chemical, but he did not read them. Id. at 1108. And testimony from Dr. Kilburn
had previously been excluded in at least six other toxic-tort cases. Id. at 1093.

        Dr. Moreno’s testimony here is far stronger than the testimony of Dr. Kilburn in
Downs. His analysis also materially contrasts with that of experts whose differential-
diagnosis testimony has been excluded in earlier Sixth Circuit cases. In Conde v.
Velsicol Chemical Corp., 24 F.3d 809 (6th Cir. 1994), for example, Dr. James Conde
was a general practitioner and the lead plaintiff in the case. Id. at 813. He testified that
chlordane, a termiticide that was applied to the basement of his home, caused various
ailments in himself and his family members. Id. Dr. Conde’s opinion was discredited
because “[n]ineteen epidemiologic studies in humans ha[d] found little evidence of long-
term adverse health effects from chlordane doses hundreds of times higher than those the
Condes were subjected to under a worst-case scenario.” Id. at 813-14.

        The three remaining causation experts offered by the plaintiffs in Conde were
“non-medical doctors unqualified to render differential diagnoses of medical
conditions.” Id. at 813. Their opinions were not admitted because they were “unable
to exclude other potential causes for [the plantiffs’] symptoms, and their theories [were]
inconsistent with the negative chlordane test results on the Condes’ tissue and the vast
majority of the relevant, peer-reviewed scientific literature.” Id. at 814. Although Dr.
Moreno did not provide any study concluding that Aqua EZ can cause anosmia, he did
discover that it could damage nasal and sinus mucosa. The record reveals no studies
comparable to those in Conde that discredit Dr. Moreno’s reasonable conclusion that a
chemical insult to the sinuses can lead to anosmia.
No. 08-5924         Best v. Lowe’s Home Centers, Inc.                              Page 17


        Two unpublished Sixth Circuit cases that excluded medical-causation testimony
founded upon differential diagnoses are also distinguishable. One of these is Rolen v.
Hansen Beverage Co., 193 F. App’x 468 (6th Cir. 2006), where a doctor opined that a
juice drink manufactured by the defendant had caused the plaintiff’s stomach problems.
The doctor employed a differential diagnosis, but presented no evidence that the juice
drink was harmful in any way other than the fact that the plaintiff fell ill shortly after
consuming it. Id. at 470-71. This conclusion was extremely dubious in light of the fact
that, despite ample opportunity to do so, the plaintiff had not tested the drink in question
for bacteria. The defendant had a juice box from the same batch (provided by the
plaintiffs) tested by an independent laboratory, which concluded that it contained “no
organisms of public health concern.” Id. at 469-70.

        Kolesar v. United Agriproducts, Inc., 246 F. App’x 977 (6th Cir. 2007), is the
other unpublished case involving a differential-diagnosis opinion that fell short of the
one developed by Dr. Moreno. In that case, the testifying physician opined that a
chemical spill caused the plaintiff’s Reactive Airways Dysfunction Syndrome (RADS).
Id. at 979. But the plaintiff suffered from asthma and a serious preexisting “smoker’s
cough”—possible causes of RADS that the doctor did not consider. Id. at 981. No such
unconsidered alternative causes of Best’s alleged anosmia have been identified in the
case before us.

        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.
No. 08-5924        Best v. Lowe’s Home Centers, Inc.                            Page 18


G.     Need for medical expert testimony

       We further note that, even without Dr. Moreno’s expert testimony, summary
judgment might be inappropriate in this case in light of this court’s recent decision in
Gass v. Marriott Hotel Services, 558 F.3d 419, 434 (6th Cir. 2009) (holding that expert
testimony was not required to prove the causation element of a negligence case where
the plaintiffs were allegedly exposed to pesticides and immediately developed
respiratory injuries). Because we conclude that Dr. Moreno’s opinion is admissible,
however, we have no need to decide whether the holding in Gass is applicable to the
present case.

                                III. CONCLUSION

       For all of the reasons set forth above, we REVERSE the judgment of the district
court and REMAND the case for further proceedings consistent with this opinion.
