Expert witnesses: qualifications, reasoning, and disclosure

Most medical malpractice cases depend on expert testimony because the standard of care and the consequences of a departure are outside ordinary lay knowledge. The useful question is not simply whether an expert has impressive credentials. It is whether the expert is qualified to address the specific issue, has considered the relevant evidence, and can explain a medically and legally sufficient conclusion.

Match the expert to the disputed question

Evidence Code section 720 recognizes qualifications through special knowledge, skill, experience, training, or education. Sections 801 and 802 govern opinion subject matter, the basis of the opinion, and examination of that basis. An expert may rely on appropriate material of a type reasonably relied upon in the field, but an unsupported assumption does not become reliable merely because an expert repeats it.

A case can require different expertise for different tasks. For example, a primary-care physician might address follow-up of a test result, a relevant specialist might address the disease's treatment and probable outcome, an informatics witness might explain routing and access, and an economist might address future financial loss. An informatics witness does not automatically establish the clinical standard, and a clinical expert does not automatically have the knowledge needed to reconstruct system configuration.

California does not impose a universal rule that every medical expert must have the identical board certification as the defendant. Qualification depends on the issue and the demonstrated foundation, subject to specific statutes. Differences in specialty, practice setting, or recent experience can nevertheless be powerful grounds to limit an opinion or challenge its weight.

The emergency-department qualification rule

Health and Safety Code section 1799.110 adds requirements for matters within its scope involving emergency medical care in a general acute care hospital emergency department. Its standard-of-care provisions account for emergency circumstances. Its expert provision, subdivision (c), directs the court in an action for negligence against a physician providing emergency medical coverage for a general acute care hospital emergency department to admit expert medical testimony only from physicians and surgeons who have had substantial professional experience within the last five years while assigned to provide emergency medical coverage in a general acute care hospital emergency department; "substantial professional experience" is determined by the custom and practice of emergency coverage in the same or similar localities.

Do not assume that occasional referrals from an emergency department satisfy this requirement. Equally, do not export the statute automatically to every urgent-care office or every medical causation opinion. The setting, the conduct at issue, the proposed testimony, and the decisions applying the statute must be examined. Zavala v. Board of Trustees and Petrou v. South Coast Emergency Group are useful authorities for this analysis.

What a reasoned opinion should show

A persuasive standard-of-care opinion identifies the defendant's role, the information reasonably available, the expected action, and the specific departure. A persuasive causation opinion identifies the counterfactual: what probably would have happened if the appropriate action had occurred at the appropriate time. It should address material competing explanations and important contrary evidence.

For a delayed-cancer-diagnosis claim, for example, an opinion should not stop at “earlier diagnosis is better.” It should address the likely disease state at the relevant earlier date, the available treatment, the patient's condition, and the probable difference in outcome. For a medication injury, it should address dose, exposure, timing, alternative causes, and the relationship between the alleged omission and the particular injury.

The same discipline applies to defense declarations. A conclusion that care was appropriate, without explaining the challenged conduct, may leave a material issue unresolved. Experts should distinguish facts established by evidence, assumptions, and inferences. If a key assumption is disputed, the effect of that dispute on the opinion should be clear.

When an expert is not required: the common-knowledge exception

Expert testimony is required in most malpractice cases because the standard of care is a matter of professional knowledge. The exception is narrow. In Flowers v. Torrance Memorial Hospital Medical Center (1994) 8 Cal.4th 992, the Supreme Court restated the rule that the standard of care against which professional acts are measured is a matter peculiarly within the knowledge of experts, and must be proved by expert testimony unless the conduct required by the particular circumstances is within the common knowledge of lay persons. A sponge left in an abdomen, or an injury to a part of the body far from the operative field, may fall within the exception or support a res ipsa loquitur inference; most diagnostic, medication and procedural judgments do not. Causation in medical cases almost always requires expert testimony even when the departure is obvious.

The court as gatekeeper

Qualification is only the first filter. In Sargon Enterprises, Inc. v. University of Southern California (2012) 55 Cal.4th 747, the Supreme Court held that trial courts have a duty, under Evidence Code sections 801 and 802, to exclude expert opinion that is based on matter of a type on which an expert may not reasonably rely, on reasons unsupported by the material on which the expert relies, or on speculation. The court does not decide which side's expert is correct; it asks whether the reasoning connects the material to the conclusion. In malpractice practice, Sargon challenges usually target causation opinions that leap from an association, a population statistic or a temporal sequence to a conclusion about this patient without a stated mechanism.

Expert declarations at summary judgment

Many malpractice cases are decided on a defense motion for summary judgment supported by an expert declaration that the care met the standard of care. The plaintiff must then produce a competing expert declaration that creates a triable issue. Courts examine the substance of both. Kelley v. Trunk (1998) 66 Cal.App.4th 519 reversed a summary judgment granted on a defense declaration that stated only an ultimate opinion; the court explained that summary judgment may not be granted on conclusory expert declarations that provide only an ultimate opinion unsupported by reasoned explanation. The same principle applies to plaintiffs' declarations: an expert who says "the delay caused the injury" without explaining how will not defeat a well-supported motion. Jennings v. Palomar Pomerado Health Systems and Bushling v. Fremont Medical Center, both summarized in the case atlas, illustrate the causation side of the problem.

Basis testimony and case-specific hearsay

Experts in malpractice cases rely heavily on out-of-court material: chart entries, other clinicians' notes, deposition testimony and published literature. In People v. Sanchez (2016) 63 Cal.4th 665, the Supreme Court held that an expert may not relate case-specific out-of-court statements to the jury as true and as the basis of an opinion unless those statements are independently proven by competent evidence or fall within a hearsay exception; general background knowledge in the expert's field is treated differently. Sanchez arose in a criminal case, but its hearsay analysis rests on the Evidence Code, which also governs civil trials. The practical lesson for both sides is to lay the foundation for the medical records and testimony an expert relies on — through custodians, business-records foundations and the witnesses themselves — rather than expecting the expert to carry the facts into evidence.

Treating physicians and retained experts

A treating physician may testify about the care provided and about opinions formed during treatment, and is often the most persuasive witness on what was observed. When a treating physician is asked to give opinions formed for the litigation — for example, about another clinician's standard of care — the witness's role changes and so may the disclosure requirements. Both sides should decide early whether a treating physician will be offered only as a percipient witness or also as a retained expert, and disclose accordingly.

What makes an opinion vulnerable

Disclosure is a separate obligation

Code of Civil Procedure sections 2034.210 and following govern expert witness exchange in California civil cases. A timely demand initiates specified disclosure obligations. Retained expert declarations, the subjects of testimony, availability for deposition, fees, and required production must be handled under the applicable provisions. Treating clinicians are not automatically exempt from disclosure; the witness's role and the opinions sought affect the requirements.

Under section 2034.300, unreasonable failures to comply with specified disclosure obligations can lead to exclusion when the statutory conditions are met. An expert's qualifications and the correctness of an opinion do not cure every disclosure failure. Supplemental or replacement opinions also require attention to the governing rules and orders.

Sources

• Evidence Code § 720, § 801, and § 802, official code entry.
• Health and Safety Code § 1799.110; EMSA statutory compilation.
• Zavala v. Board of Trustees (1993) 16 Cal.App.4th 1755: opinion; Petrou v. South Coast Emergency Group (2004) 119 Cal.App.4th 1090: opinion.
• Jennings v. Palomar Pomerado Health Systems, Inc. (2003) 114 Cal.App.4th 1108: opinion.
• Flowers v. Torrance Memorial Hospital Medical Center (1994) 8 Cal.4th 992: opinion (Justia reproduction); Kelley v. Trunk (1998) 66 Cal.App.4th 519: opinion (Justia reproduction); People v. Sanchez (2016) 63 Cal.4th 665: opinion (Justia reproduction).
• Sargon Enterprises, Inc. v. University of Southern California (2012) 55 Cal.4th 747, summarized with a link in the case atlas.
• Code of Civil Procedure § 2034.220 and § 2034.230; official entries for § 2034.260 and § 2034.300.