On July 23, 2023, California Senate Bill 652 was signed into law, changing the standard of proof required for admissibility of expert opinion testimony regarding alternative medical causation at trial. The new law, which was drafted in response to a California Court of Appeal decision that affirmed the existing standard of proof for admission of defense expert medical opinions at trial, imposes a higher standard of proof on defendants. This amendment to the California Evidence Code, under new section 801.1 goes into effect in 2024.

Under existing law, a defense medical expert offering trial testimony regarding alternative causes of a plaintiff’s injury is permitted to testify as to those opinions even where those opinions fail to meet the standard of proof applicable to the plaintiff’s medical experts – that the opinion offered is supported by a reasonable degree of medical probability. In other words, prior to the change in law, defense experts were permitted to testify as to medically potential or possible causes of a plaintiff’s injury other than the alleged acts of the defendant, rather than being required to testify that the alternative cause of injury was to a degree of medical probability.

In 2022, a California Court of Appeal opinion affirmed the existing admissibility standard for medical expert opinion under Evidence Code section 801. In Kline v. Zimmer, Inc. (2022) 79 Cal.App.5th 123, the court found that the reasonable probability requirement applies only to the party bearing the burden of proof on the underlying question of medical causation, i.e., usually the plaintiff.

In response to this decision, SB 652 was introduced at the behest of advocates for the California plaintiffs’ bar to override the Kline decision and impose on defendants a higher standard for admissibility of expert testimony on medical causation. Under the new law, all expert testimony regarding medical causation must be supported by the reasonable degree of medical probability standard, regardless of which party bears the burden of proof in the case. Defense experts unable to support their opinions to that heightened standard will be unable to testify as to those opinions.

Codified as California Evidence Code section 801.1, the evidence code will read:

801.1. (a) Where the party bearing the burden of proof proffers expert testimony regarding medical causation and where that party’s expert is required as a condition of testifying to opine that causation exists to a reasonable medical probability, the party not bearing the burden of proof may offer a contrary expert only if its expert is able to opine that the proffered alternative cause or causes each exists to a reasonable medical probability, except as provided in subdivision (b).

(b) Subdivision (a) does not preclude a witness testifying as an expert from testifying that a matter cannot meet a reasonable degree of probability in the applicable field, and providing the basis for that opinion.

Notwithstanding the change in the standard for admissibility for expert testimony brought about by Evidence Code section 801.1(a), it is worth noting that section 801.1(b) permits experts to explain why an opinion on medical causation offered by an opposing expert does not meet the reasonable degree of probability standard. We expect that experts on both sides of a case will make use of this provision.

The change in the Evidence Code will require counsel to work closely with their medical experts to ensure their opinions on causation are articulated with the higher admissibility standard in mind and, to the extent that experts believe that an opposing expert’s opinions are not supportable to the heightened standard, experts should be prepared to offer that opinion and the support for same.

As a practice point, defense counsel would be wise to consider whether the heightened admissibility standard for medical causation testimony provides additional support for an argument to the court for the right to conduct more expansive discovery into a plaintiff’s medical history.

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