Treating Physician Testimony on Causation in California PI Cases

Treating physicians carry a level of credibility that retained experts can’t replicate. Their opinions are rooted in firsthand patient care, not litigation strategy. That credibility makes their testimony invaluable for linking the incident to the injury, confirming treatment necessity, and establishing the “reasonable certainty” required for future medical expenses.

But this testimony isn't automatic. It is bounded by procedural and evidentiary rules that most treaters are unfamiliar with and defense counsel know how to exploit. Mastering the standards that govern treating physicians’ opinions is critical to keeping this testimony in evidence.

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Treating Physicians as Non-Retained Experts

California distinguishes between:

1. Retained experts — hired to provide opinions for litigation, and
2. Non-retained experts — whose opinions are based on facts “independently acquired” in their professional work.

Treating physicians generally fall into the second category.

In Ochoa v. Dorado (2014) 228 Cal.App.4th 120, 140, the court held that a treating physician may testify to “facts acquired independently of the litigation” during the physician-patient relationship. This includes diagnosis, prognosis, and  medical causation.

The California Supreme Court confirmed in Schreiber v. Estate of Kiser (1999) 22 Cal.4th 31, 39, that causation opinions are “inherent in a physician’s work.” A treater may testify that the incident caused the plaintiff’s injury even without reviewing pre-incident records, provided the opinion is grounded in examination, treatment, and professional experience.

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The Legal Standard for Causation Opinions

The concept of legal causation can be confusing for medical experts. While they know the medical side, they are often unfamiliar with California’s legal standards. As the attorney, you must use the correct language when questioning causation witnesses so their answers satisfy California law. The Judicial Council of California Civil Jury Instructions (CACI) provide the framework — and often the exact language — to use in your questioning.

Burden of Proof (CACI 200). Explain the civil standard of proof as “more likely than not.” Use analogies: balanced scales, or the 51% confidence rule. Contrast it with the higher “beyond a reasonable doubt” standard in criminal cases to help experts understand.

More Likely Than Not. Causation in a personal injury action must be proven within a “reasonable medical probability” based on competent expert testimony. (Jones v. Ortho Pharmaceutical Corp. (1985) 163 Cal.App.3d 396, 403.) A possible cause becomes “probable” when it is more likely than not that the event caused the injury.

Substantial Factor Test (CACI 430). Under this test, a cause must be more than remote or trivial, but need not be the only cause. It subsumes the “but-for” test, where an act is a cause if the injury would not have occurred but for that act. (Rutherford v. Owens-Illinois, Inc. (1997) 16 Cal.4th 953, 968-969.)

Multiple Causes (CACI 431). Prepare your expert for cross-examination suggesting other causes. Explain that a defendant’s negligence can combine with other factors to cause harm—and that the presence of another substantial factor does not absolve the defendant.

Eggshell Plaintiff Rule (CACI 3928). If your client has a pre-existing condition making them more vulnerable to injury, remind your expert that the defendant “takes the plaintiff as they find them.” (Rideau v. Los Angeles Transit Lines (1954) 124 Cal.App.2d 466, 471.) The defendant remains liable for the full extent of harm, even if it is greater due to the plaintiff’s condition.

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Impact of Evidence Code § 801.1

California’s Evidence Code § 801.1 reinforces an equal standard for both sides when it comes to medical causation opinions:

• Equal Standard for All: Both plaintiff and defense experts must adhere to the “reasonable medical probability” standard when offering medical causation opinions. Neither side can lower the threshold for its own witnesses.
• Alternative Causes: If a defense expert proposes an alternative cause for the injury, they must also testify that the alternative cause exists to a reasonable medical probability — not just that it is possible. This prevents speculative “maybe it was something else” testimony from influencing the jury.

Practice Tip: When preparing your treating physician, ensure their causation opinions are framed in terms of “reasonable medical probability.”

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Defense Traps — and How to Avoid Them

Defense counsel strategies to keep out causation opinions include:

• If the treater hasn’t seen prior medical records — claim they can’t rule out pre-existing conditions.
• If the treater has seen them — argue they became a retained expert requiring designation.

The law doesn’t support either extreme. Treaters may base opinions solely on their treatment experience (Schreiber, supra). They may also review prior records without automatically becoming retained — as long as the purpose is not to form litigation opinions.

But Dozier v. Shapiro (2011) 199 Cal.App.4th 1509 warns: giving a treater significant outside materials for litigation purposes can convert them into a retained expert. To stay safe:

• Disclose the treater as a “retained treating physician” if you plan to provide substantial additional records or hypotheticals beyond treatment scope.
• Give notice and allow the defense to depose on those expanded opinions.

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Sanchez and Laying Foundation

In People v. Sanchez (2016) 63 Cal.4th 665, 686, the Court barred experts from relating case-specific hearsay as true unless independently proven or covered by a hearsay exception. This directly affects damages proof when a retained expert relies on treater diagnoses.

Solution: Have the treater lay the foundation under the business records exception (Evid. Code § 1271) by authenticating their own opinions, diagnoses, notes, operative reports, and imaging. Once those facts and opinions are in evidence, retained experts can build on them without running afoul of Sanchez.

Example: If your orthopedic expert projects a future knee replacement based on a treating orthopedist’s diagnosis of post-traumatic arthritis, you must get that diagnosis into evidence through the treater.

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Proving Future Treatment Needs — CACI 3903A

To recover future medical expenses, you must prove:

1. The plaintiff is reasonably certain to need the care in the future.
2. The reasonable cost of that care.
3. That the care is reasonably necessary because of the incident.
   (CACI 3903A; Bellman v. San Francisco High School Dist. (1938) 11 Cal.2d 576, 588.)

Treaters are uniquely positioned to satisfy the “reasonable certainty” prong because they have followed the patient’s course and prognosis. In Markow v. Rosner (2016) 3 Cal.App.5th 1027, 1050, the court upheld CACI 3903A’s instruction that the jury determine the reasonable cost of care the plaintiff “is reasonably certain to need in the future.”

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Practical Deposition and Trial Tips

• Prep with Purpose: Review key chart notes and surgical reports; confirm opinions on causation, permanence, and future care.
• Show, Don’t Just Tell: Use X-rays, surgical hardware, and medical illustrations; have the treater confirm accuracy.
• Lay the Foundation: Establish record authenticity and creation in the ordinary course of business.
• Be Ready for Objections: When challenged as “outside scope” or “calls for expert opinion,” cite Schreiber and Ochoa.

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Hypotheticals as a Foundation Tool

If a treater hesitates on causation for lack of prior history, use hypotheticals rooted in admitted evidence. (Rosenberg v. Goldstein (1966) 247 Cal.App.2d 25, 30). This lets them consider facts beyond their treatment while remaining within permissible opinion testimony.

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Conclusion

Treating physicians occupy a powerful evidentiary position in California injury cases. They can give causation, injury, and future care opinions with a credibility jurors instinctively trust; but only if you respect the boundaries between non-retained and retained status, ensure compliance with the "reasonable medical probability" standard, Sanchez, and Evidence Code § 801.1, and tie their future treatment opinions to CACI’s requirements.

Done right, your client’s treating doctors won’t just support your damages case—they’ll help you carry it across the finish line.

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Conor Hulburt is a plaintiff’s trial attorney and founder of the Hulburt Law Firm in San Diego, California. Conor has recovered over $100 million on behalf of clients in catastrophic injury and wrongful death cases. He has successfully taken on Fortune 500 companies, major auto carriers, and government entities, including a landmark $28.1 million jury verdict in Quiroz v. Caltrans.