Medical expert’s opinion based on process of elimination was improperly excluded from trial, appellate court holds

Timothy Kowal, Esq.
October 26, 2022

Sometimes it is hard to pinpoint what actually caused a harm, like a medical injury. But we can use the process of elimination. An opthalmologist expert offered an opinion based on the process of elimination—differential etiology, in medical jargon. But the trial court excluded it, and then granted the defendant hospital’s motion for nonsuit.

That was an abuse of discretion, held the appellate court in Siemon v. Regents of the University of California (D1d1 Oct. 19, 2022 no. A160654) 2022 WL 12083207 (nonpub. opn.). Differential etiology—i.e., process of elimination—is a valid method of establishing proximate causation, so long as the jury finds it credible.

The plaintiff in Siemon suffered injury after eye surgery. The injury was an inflammatory reaction known as toxic anterior segment syndrome, or TASS, occurring after eye surgery and resulting in inflammation, pain, and possible vision loss. The plaintiff’s opthalmologist expert opined the cause was inadequately cleaned and sterilized instruments used during surgery.

But the expert did not identify any actually contaminated instruments. Instead, the plaintiff’s expert eliminated all other variables so that improper sterilization was “the only cause of TASS remaining.”

The trial court granted the defendant hospital’s motion to exclude the expert’s opinion, and then granted the hospital’s motion for nonsuit following the plaintiff’s opening argument.

Process of elimination is a valid basis for an expert’s opinion on causation.

The Court of Appeal reversed. The plaintiff’s expert’s opinion was based on a valid method of determining causation.

“ ‘Differential diagnosis, or differential etiology, is a standard scientific technique of identifying the cause of a medical problem by eliminating the likely causes until the most probable one is isolated.... [Citation.] ... [¶] The first step in the diagnostic process is to compile a comprehensive list of hypotheses that might explain the set of salient clinical findings under consideration. [Citation.] The issue at this point in the process is which of the competing causes are generally capable of causing the patient's symptoms or mortality .... [¶] After the expert rules in all of the potential hypotheses that might explain a patient's symptoms, he or she must then engage in a process of elimination, eliminating hypotheses on the basis of a continuing examination of the evidence so as to reach a conclusion as to the most likely cause of the findings in that particular case.’ ” (Johnson & Johnson Talcum Powder Cases (2019) 37 Cal.App.5th 292, 308, fn. 6 (Johnson & Johnson).)

The plaintiff need not establish “ ‘the proximate cause of injury with absolute certainty so as to exclude every other possible cause of a plaintiff's illness, even if the expert's opinion was reached by performance of a differential diagnosis.’ [Citation.] Instead, ‘ “the plaintiff must offer an expert opinion that contains a reasoned explanation illuminating why the facts have convinced the expert, and therefore should convince the jury, that it is more probable than not” ’ that the product was a cause-in-fact of the disease. [Citation.] Then the burden shifts to the defendant to prove ‘the existence of an alternative explanation, supported by substantial evidence and not mere speculation,’ to defeat the plaintiffs’ explanation as a matter of law.” (Pilliod v. Monsanto Co. (2021) 67 Cal.App.5th 591, 623.)

Here, the hospital did not dispute that differential etiology analysis was valid. Instead, the hospital argued the plaintiff’s expert did not conduct a valid differential etiology analysis because he did not consider the full range of potential causes of TASS and failed to adequately rule out possible causes. But the hospital did not point to any other causes that the plaintiff’s expert failed to consider.

But the process of elimination is limited because it cannot prove negligence.

The trial court also prohibited the plaintiff’s expert from testifying that the hospital “failed” to properly clean or sterilize instruments. Even though the Court of Appeal held that the expert should have been permitted to opine that dirty instruments caused the plaintiff’s TASS injury, that does not mean the dirty instruments were the result of the hospital’s “failure.” An instrument can become dirty in the absence of negligence.

As the plaintiff’s expert acknowledged, “even if an instrument is 100 percent sterilized there could be residual dead bacteria whose wall has an endotoxin that can precipitate TASS.”

For the same reason, res ipsa loquitur did not apply here. There were other ways TASS could have resulted, or even that the instruments could have been dirty, that did not include the defendant hospital’s negligence.

The Court of Appeal reversed with directions to conduct a new trial allowing the plaintiff’s expert to testify that the TASS was likely caused by a contaminated instrument, and that generally this is a breach of the standard of care. But the expert may not testify that the hospital actually failed to properly clean or sterilize any instruments.


These close legal calls on expert evidence are often made during the trial, and the parties have to go through trial, judgment, postjugment motions, and appeals before knowing whether the key evidence in the case stays in or comes out.

Here, the trial court made the call before trial began, and the Court of Appeal weighed in on the call after a nonsuit. When the parties start trial again, the plaintiff will be armed with the Court of Appeal’s observation that the plaintiff’s evidence “would be sufficient to support a judgment in [her] favor.” That counts for something.

Tim Kowal is an appellate specialist certified by the California State Bar Board of Legal Specialization. Tim helps trial attorneys and clients win their cases and avoid error on appeal. He co-hosts the Cal. Appellate Law Podcast at, and publishes a newsletter of appellate tips for trial attorneys at Contact Tim at or (714) 641-1232.

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