A recent decision of the California Court of Appeal, Second District, Division Three, continues the dubious state and federal practice of deciding important issues under cover of unpublished opinions. Cooper v. Takeda Pharms. America, Inc., No. B250163, 2015 Cal. App. Unpub. LEXIS 4965 (Calif. App., 2nd Dist., Div. 3; July 16, 2015). In Cooper, plaintiff claimed that her late husband’s bladder cancer was caused by defendant’s anti-diabetic medication, Actos (pioglitazone). The defendant moved to strike the expert witness testimony in support of specific causation. The trial judge expressed serious concerns about the admissibility of plaintiff’s expert witnesses on specific causation, but permitted the trial to go forward. After a jury returned its verdict in favor of plaintiff, the trial court entered judgment for the defendants, on grounds that the plaintiff lacked admissible expert witness testimony.
Although a recent, large, well-conducted study failed to find any meaningful association between pioglitazone and bladder cancer, there were, at the time of trial, several studies that suggested an association. Plaintiff’s expert witnesses, epidemiologist Dr. Alfred Neugut and bladder oncologist Dr. Norm Smith interpreted the evidence to claim a causal association, but both conceded that there were no biomarkers that allowed them to attribute Cooper’s cancer to pioglitazone. The plaintiff also properly conceded that identifying a cause of the bladder cancer was irrelevant to treating the disease. Cooper, 2015 Cal. App. Unpub. LEXIS 4965, at *13. Specific causation was thus determined by the so-called process of differential etiology, with the ex ante existence of risk substituting for cause, and using risk exposure in the differential analysis.
The trial court was apparently soured on Dr. Smith’s specific causation assessment because of his poor performance at deposition, in which he demonstrated a lack of understanding of Cooper’s other potential exposures. Smith’s spotty understanding of Cooper’s actual and potential exposures and other risks made any specific causation assessment less than guesswork. By the time of trial, Dr. Smith and plaintiff’s counsel had backfilled the gaps, and Smith presented a more confident analysis of Cooper’s exposures and potentially competing risks.
Cooper had no family history of bladder cancer, no alcohol consumption, and no obvious exposure to occupational bladder carcinogens. His smoking history would account for exposure to a known bladder carcinogen, cigarette smoke, but Cooper’s documented history was of minor tobacco use, and remote in time. Factually, Cooper’s history was suspect and at odds with his known emphysema. Based upon this history, along with their causal interpretation of the Actos bladder cancer association, and their quantitative assessment that the risk ratio for bladder cancer from Actos was 7.0 or higher for Mr. Cooper (controlled for covariate, potential confounders), the plaintiff’s expert witnesses opined that Actos was probably a substantial factor in causing Mr. Cooper’s bladder cancer. The court did not examine the reasonableness of Dr. Smith’s risk ratios, which seem exorbitant in view of several available meta-analyses.
The court stated that under the applicable California law of “substantial factor,” the plaintiff’s expert witness, in conducting a differential diagnosis, need not exclude every other possible cause of plaintiff’s disease “with absolute certainty.” Cooper, at *41-42. This statement leaves unclear and ambiguous whether the plaintiff’s expert witness must (and did in this case) rule out other possible causes with some level of certitude less than “absolute certainty,” such as reasonable medical certainty, or perhaps reasonable probability. Dr. Smith’s testimony, as described, did not attempt to go so far as to rule out smoking as “a cause” of Cooper’s bladder cancer; only that the risk from smoking was a lower order of magnitude than that for Actos. In Dr. Smith’s opinion, the discrepancy in magnitude between the risk ratios for smoking and Actos allowed him to state confidently that Actos was the most substantial risk.
Having estimated the smoking-related increased risk to somewhere between 0 and 100%, with the Actos increased risk at 600% or greater, Dr. Smith was able to present an admissible opinion that Actos was a substantial factor. Of course, this all turns on the appellate court’s acceptance of risk, of some sufficiently large magnitude, as evidence of specific causation. In the Cooper court’s words:
“The epidemiological studies relied on by Dr. Smith indicated exposure to Actos® resulted in hazard ratios for developing bladder cancer ranging from 2.54 to 6.97.18 By demonstrating a relative risk greater than 2.0 that a product causes a disease, epidemiological studies thereby become admissible to prove that the product at issue was more likely than not responsible for causing a particular person’s disease. “When statistical analyses or probabilistic results of epidemiological studies are offered to prove specific causation . . . under California law those analyses must show a relative risk greater than 2.0 to be ‘useful’ to the jury. Daubert v. Merrell Dow Pharmaceuticals Inc., 43 F.3d 1311, 1320 (9th Cir.), cert. denied 516 U.S. 869 (1995) [Daubert II]. This is so, because a relative risk greater than 2.0 is needed to extrapolate from generic population-based studies to conclusions about what caused a specific person’s disease. When the relative risk is 2.0, the alleged cause is responsible for an equal number of cases of the disease as all other background causes present in the control group. Thus, a relative risk of 2.0 implies a 50% probability that the agent at issue was responsible for a particular individual’s disease. This means that a relative risk that is greater than 2.0 permits the conclusion that the agent was more likely than not responsible for a particular individual’s disease. [Reference Manual on Scientific Evidence (Federal Judicial Center 2d ed. 2000) (“Ref. Manual”),] Ref. Manual at 384, n. 140 (citing Daubert II).” (In re Silicone Gel Breast Implant Prod. Liab. Lit. (C.D. Cal. 2004) 318 F.Supp.2d 879, 893; italics added.) Thus, having considered and ruled out other background causes of bladder cancer based on his medical records, Dr. Smith could conclude based on the studies that it was more likely than not that Cooper’s exposure to Actos® caused his bladder cancer. In other words, because the studies, to varying degrees, adjusted for race, age, sex, and smoking, as well as other known causes of bladder cancer, Dr. Smith could rely upon those studies to make his differential diagnosis ruling in Actos®—as well as smoking—and concluding that Actos® was the most probable cause of Cooper’s disease.”
Cooper, at *78-80 (emphasis in the original).
Of course, the epidemiologic studies themselves are not admissible, regardless of the size of the relative risk, but the court was, no doubt, speaking loosely about the expert witness opinion testimony that was based upon the studies with risk ratios greater than two. Although the Cooper case does not change California law’s facile acceptance of risk as a substitute for cause, the case does base its approval of plaintiff’s expert witness’s attribution as turning on the magnitude of the risk ratio, adjusted for confounders, as having exceeded two. The Cooper case leaves open what happens when the risk that is being substituted for cause is a ratio ≤ 2.0. Some critics of the risk ratio > 2.0 inference have suggested that risk ratios greater than two would lead to directed verdicts for plaintiffs in all cases, but this suggestion requires demonstrations of both the internal and external validity of the studies that measure the risk ratio, which in many cases is in doubt. In Cooper, the plaintiff’s expert witnesses’ embrace of a high, outlier risk ratio for Actos, while simultaneously downplaying competing risks, allowed them to make out their specific causation case.
 James D. Lewis, Laurel A. Habel, Charles P. Quesenberry, Brian L. Strom, Tiffany Peng, Monique M. Hedderson, Samantha F. Ehrlich, Ronac Mamtani, Warren Bilker, David J. Vaughn, Lisa Nessel, Stephen K. Van Den Eeden, and Assiamira Ferrara, “Pioglitazone Use and Risk of Bladder Cancer and Other Common Cancers in Persons With Diabetes,” 314 J. Am. Med. Ass’n 265 (2015) (adjusted hazard ratio 1.06, 95% CI, 0.89-1.26).
 See, e.g., R.M. Turner, et al., “Thiazolidinediones and associated risk of bladder cancer: a systematic review and meta-analysis,” 78 Brit. J. Clin. Pharmacol. 258 (2014) (OR = 1.51, 95% CI 1.26-1.81, for longest cumulative duration of pioglitazone use); M. Ferwana, et al., “Pioglitazone and risk of bladder cancer: a meta-analysis of controlled studies,” 30 Diabet. Med. 1026 (2013) (based upon 6 studies, with median follow-up of 44 months, risk ratio = 1.23; 95% CI 1.09-1.39); Cristina Bosetti, “Cancer Risk for Patients Using Thiazolidinediones for Type 2 Diabetes: A Meta-Analysis,” 18 The Oncologist 148 (2013) (RR = 1.64 for longest exposure); Shiyao He, et al., “Pioglitazone prescription increases risk of bladder cancer in patients with type 2 diabetes: an updated meta-analysis,” 35 Tumor Biology 2095 (2014) (pooled hazard ratio = 1.67 (95% C.I., 1.31 – 2.12).