Reversing Exclusion Of Medical Expert Differential Causation Testimony

Ninth Circuit notes that while medical expert testimony may lack certainty it may aid the jury in determining material issues of fact; a consensus has emerged among the circuits on the use of differential diagnosis; the district court erred in concluding the expert testimony was irrelevant and unreliable, in Messick v. Novartis Pharmaceuticals Corp., _ F.3d _ (9th Cir. April 4, 2013) (No. 13-15433)

Medical expert testimony is often important to assist the jury in deciding causation issues. Unlike other sciences, medical opinions are often not capable of precision or certainty. One significant area of medical opinion testimony involves differential diagnosis. As the Ninth Circuit has noted:

A whole sub-body of Daubert law has developed with respect to the reliability, and admissibility, of differential diagnosis. Differential diagnosis is "the determination of which of two or more diseases with similar symptoms is the one from which the patient is suffering, by a systematic comparison and contrasting of the clinical findings." Stedman's Medical Dictionary 474 (26th ed. 1995).

Clausen v. M/V NEW CARISSA, 339 F.3d 1049, 1057 (9th Cir. 2003). A recent Ninth Circuit case considered the admissibility of expert testimony involving differential diagnosis. The case is instructive in considering the role of medical expert testimony.

Trial Court Exclusion Of Medical Causation Expert

In the case, plaintiff Messick was diagnosed with breast cancer. After being treated with the drug Zometa, which is produced by defendant Novartis Pharmaceuticals Corporation, she was diagnosed with bisphosphonate-related osteonecrosis of the jaw (“BRONJ”). According to the American Association of Oral and Maxillofacial Surgeons (“AAOMS”), the condition "lasts more than eight weeks and is not related to radiation therapy." Messick, _ F.3d at _. Three years after her diagnosis, the plaintiff was healed. Along with her husband, she filed an action against defendant Novartis alleging claims for strict products liability, negligent manufacture, negligent failure to warn, breach of express and implied warranty, and loss of consortium.

The defendant moved for summary judgment. The district court granted summary judgment after excluding the plaintiff’s medical causation expert, Dr. Jackson, as offering irrelevant and unreliable testimony. According to the district court, Dr. Jackson’s “differential diagnosis only determines that Ms. Messick’s ONJ is related to her bisphosphonate use, and he admits that a diagnosis of BRONJ does not mean that bisphosphonates caused her ONJ.” The expert opined “that a patient without cancer or exposure to radiation in the mouth area would not develop ONJ lasting for years without IV bisphosphonate treatments.” The court concluded that the expert “never explained the scientific basis for this conclusion." Messick, _ F.3d at _, _. After summary judgment was granted for the defendant, the plaintiff appealed the exclusion of the medical expert evidence and grant of summary judgment.

Ninth Circuit Reversal

The Ninth Circuit concluded that the district court erred in excluding the medical expert on irrelevancy and unreliability grounds.

First, the circuit found that the “district court applied too high a relevancy bar without reference to California law.” In particular, “California state products liability law requires only that a plaintiff show that the defendant’s conduct was ‘more likely than not’ a substantial factor in causing the injury in order to prove specific causation.” Messick, _ F.3d at _ (citing Saelzler v. Advanced Grp. 400, [25 Cal.4th 763, 107 Cal. Rptr. 2d 617], 23 P.3d 1143, 1152 (Cal. 2001)). While the medical expert “never explicitly stated that Messick’s bisphosphonate use caused her BRONJ, he did say it was at least a substantial factor in her development of BRONJ.” In his words, he “equated Messick’s use of bisphosphonates leading to BRONJ with the oxygen necessary to start a fire.” Because he opined that the use of bisphosphonate “was a substantial factor in her development of BRONJ,” the medical testimony was relevant. Messick, _ F.3d at _ (citing Daubert v. Merrell Dow Pharm., Inc., 43 F.3d 1311, 1315 (9th Cir. 1995) (“Daubert II”) (expert relevancy requires that the evidence “logically advances a material aspect of the proposing party’s case”)).

The circuit also found error in the district court’s conclusion that the expert testimony was unreliable. In applying differential diagnosis, when a potential cause is excluded, the “expert must provide reasons for rejecting alternative hypotheses using scientific methods and procedures and the elimination of those hypotheses must be founded on more than subjective beliefs or unsupported speculation.” Messick, _ F.3d at _ (quoting Clausen, 339 F.3d at 1058 (internal quotation marks and citation omitted); Daubert II, 43 F.3d at 1318 (requiring explanations based on “scientifically valid principles”).

The circuit disagreed that the expert failed to provide a “scientific basis” for his “conclusion.” Instead, his opinion was based on “his own extensive clinical experience as the basis for his differential diagnosis, as well as his examination of Messick’s records, treatment, and history” as well as “the AAOMS definition of BRONJ.”

Medical opinions often are not capable of certainty. As the circuit noted, “Although experts must provide scientifically sound reasons for excluding potential causes, we have consistently recognized the difficulties in establishing certainty in the medical sciences." Messick, _ F.3d at _ (citing Primiano v. Cook, 598 F.3d 558, 565 (9th Cir. 2010) (stating that “medical knowledge is often uncertain") (quoting United States v. Sandoval-Mendoza, 472 F.3d 645, 655 (9th Cir. 2006); see also Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579, 590 (1993) (noting that “it would be unreasonable to conclude that the subject of scientific testimony must be ‘known’ to a certainty; arguably, there are no certainties in science”); Kennedy v. Collagen Corp., 161 F.3d1226, 1230 (9th Cir. 1998) (“Causation can be proved even when we don’t know precisely how the damage occurred, if there is sufficiently compelling proof that the agent must have caused the damage somehow.”) (quoting Daubert II, 43 F.3d at 1314)). In fact, even the AAOMS had noted in 2009 the inability to establish causality but “emerging experimental and epidemiologic studies have established a firm foundation for a strong association between monthly IV bisphosphonate therapy and the development of BRONJ.”

As the circuit explained:

Because of that inherent uncertainty, we do not require that an expert be able to identify the sole cause of a medical condition in order for his or her testimony to be reliable. It is enough that a medical condition be a substantial causative factor. We hold that the district court abused its discretion in excluding Dr. Jackson’s causation testimony when it found that testimony to be unreliable largely because Dr. Jackson could not “determine in a patient who has multiple risk factors at one time which of those particular risk factors is causing [the ONJ].” Such an unduly exacting standard goes beyond the district court’s proper gatekeeping role. Dr. Jackson’s testimony should be admitted as relevant and reliable. Remaining issues regarding the correctness of his opinion, as opposed to its relevancy and reliability, are a matter of weight, not admissibility. Kennedy, 161 F.3d at 1230–31.

Messick, _ F.3d at _. The medical expert testimony could have assisted the jury in determining a material issue in fact. As the circuit concluded, “A doctor using a differential diagnosis grounded in significant clinical experience and examination of medical records and literature can certainly aid the trier of fact and cannot be considered to be offering ‘junk science.’” Messick, _ F.3d at _.

Circuit Consensus On Differential Diagnosis

The Ninth Circuit noted that it and other circuits “have held that a reliable differential diagnosis may form the basis of an expert’s causation testimony,” citing to the following cases:

  • First Circuit: Baker v. Dalkon Shield Claimaints Trust, 156 F.3d 248, 253 (1st Cir. 1998) (stating that “‘differential diagnosis’ is a standard medical technique”)
  • Second Circuit: McCullock v. H.B. Fuller Co. , 61 F.3d 1038, 1044 (2d Cir. 1995) (Medical expert testimony was based “on a range of factors” including “use of a scientific analysis known as differential etiology (which requires listing possible causes, then eliminating all causes but one); and reference to various scientific and medical treatises. Disputes as to the strength of his credentials, faults in his use of differential etiology as a methodology, or lack of textual authority for his opinion, go to the weight, not the admissibility, of his testimony.”)
  • Third Circuit: In re Paoli R.R. Yard PCD Litig. , 916 F.2d 829, 862 (3d Cir. 1990) (considering differential diagnoses performed by "non-physician experts")
  • Fourth Circuit: Cooper v. Smith & Nephew, Inc., 259 F.3d 194, 202 (4th Cir. 2001) (holding that an expert’s opinion based on a differential diagnosis is generally admissible)
  • Sixth Circuit: Hardyman v. Norfolk & W. Ry. Co., 243 F.3d 255, 260 (6th Cir. 2001) (“Differential diagnosis . . . is a standard scientific technique of identifying the cause of a medical problem”)
  • Seventh Circuit: Ervin v. Johnson & Johnson, Inc., 492 F.3d 901, 904 (7th Cir. 2007) (noting that “[a] differential diagnosis satisfies a Daubert analysis if the expert uses reliable methods”)
  • Eighth Circuit: Turner v. Iowa Fire Equip. Co., 229 F.3d 1202, 1208 (8th Cir. 2000) (“[A] medical opinion about causation, based upon a proper differential diagnosis, is sufficiently reliable to satisfy Daubert.”)
  • Ninth Circuit: Clausen v. M/V NEW CARISSA, 339 F.3d 1049, 1057 (9th Cir. 2003) ("Differential diagnosis is a common scientific technique, and federal courts, generally speaking, have recognized that a properly conducted differential diagnosis is admissible under Daubert.") (citing cases)
  • Tenth Circuit: Goebel v. Denver & Rio Grande W. R.R. Co. , 346 F.3d 987, 999 (10th Cir. 2003) (medical expert differential diagnosis testimony was admissible "because he followed a standard and accepted methodology in arriving at the diagnosis, he adequately explained why [plaintiff] Mr. Goebel might not exhibit every symptom of acute HACE, and he adequately considered and ruled out some alternative explanations even if he did not explicitly rule out depression")
  • Eleventh Circuit: McClain v. Metabolife Int’l, Inc., 401 F.3d 1233, 1252 (11th Cir. 2005) (detailing a reliable differential diagnostic process)
  • D.C. Circuit: Ambrosini v. Labarraque, 101 F.3d 129, 140 (D.C. Cir. 1996) (discussing the role of differential diagnosis to prove specific causation)

The Ninth Circuit noted that while the “Fifth Circuit is alone in not yet explicitly approving the use of differential diagnosis,” a recent case noted “that a differential diagnosis may be admissible to prove specific causation.” Messick, _ F.3d at _ (citing Johnson v. Arkema, Inc., 685 F.3d 452, 467 (5th Cir. 2012) (noting "the results of a differential diagnosis are far from reliable per se")).


The Messick case confirms some interesting aspects about medical expert testimony. First, in using medical opinion testimony, certainty is not expected. There still must be a substantial basis to support the conclusion. Generally, differential diagnosis is considered to be reliable. Second, there is a consensus among the circuits on the use of medical differential diagnosis testimony. Finally, all circuits except the Fifth Circuit have concurred on the use of differential diagnosis expert testimony.

For prior posts in the Federal Evidence Blog discussing differential diagnosis, consider:


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