To prove causation in medical malpractice cases, you have to show two things: that the provider’s error was the factual cause of your injury, and that the injury was a foreseeable result of that error. Both pieces almost always require testimony from a qualified medical expert, and you have to carry them by a preponderance of the evidence — meaning it’s more likely than not that the negligence produced the harm. Factual causation and legal causation each have their own tests, their own exceptions, and their own ways of breaking down when the medicine is complicated, and most malpractice cases are won or lost on this element rather than on whether the provider made a mistake.
Start With the But-For Test
Factual causation begins with a simple question: would you still have been injured if the provider had done things correctly? That’s the but-for test. You mentally remove the alleged error from the timeline and ask whether the harm still occurs.1Legal Information Institute. But-For Test If the injury disappears when the error disappears, you’ve established the first link in the chain. If it doesn’t, the provider isn’t the factual cause no matter how negligent the conduct was.
Some cases make this straightforward. A surgeon leaves a sponge inside a patient and the patient develops an infection at that site. Remove the sponge from the scenario and the infection doesn’t happen. The test gets harder when the patient had pre-existing conditions or was already declining. A missed cancer diagnosis forces everyone to grapple with whether earlier detection would have actually changed the outcome. Isolating the medical error from everything else going on with the patient’s health is where most of the real fight happens.
When More Than One Cause Is in Play
The but-for test has a well-known blind spot. It struggles when two or more independent acts each would have been enough to cause the harm on their own. If two providers separately prescribe medications that interact fatally, neither can say “my prescription alone wouldn’t have killed the patient” because the other was also present. Under strict but-for logic, neither caused the death, which is an absurd result.
Courts handle this with the substantial factor test. Instead of asking whether the injury would have occurred without the defendant’s conduct, this test asks whether the defendant’s conduct was a significant contributing factor in producing the harm.2Legal Information Institute. Substantial Factor Test The conduct doesn’t have to be the sole or even primary cause. It just can’t be trivial. When multiple providers each contribute to a single injury, this test lets courts hold each one accountable for the role they played rather than letting everyone escape because someone else also messed up.
Proximate Cause: Was the Harm Foreseeable?
Factual causation isn’t enough on its own. Courts also require proximate cause, which asks a different question: was this type of harm a reasonably foreseeable consequence of the provider’s negligence?3PubMed Central. An Introduction to Medical Malpractice in the United States – Section: Legal Elements of Medical Malpractice A provider answers for the kinds of injuries a competent professional would recognize as risks of the particular error, not for every misfortune that happens to follow it.
If a doctor negligently prescribes the wrong antibiotic and the patient has a foreseeable allergic reaction, that falls within the scope of the risk the doctor created. If the patient, while recovering from that reaction, is struck by lightning in the hospital parking lot, the doctor isn’t paying for the lightning strike. Proximate cause draws the line between consequences that belong to the error and consequences that are too remote or bizarre to pin on anyone.
Superseding Causes That Break the Chain
Even when a provider’s negligence sets harm in motion, an intervening event can sometimes cut off liability. Courts call this a superseding cause, and the key factor is foreseeability. If the intervening event was so unexpected and unrelated to the original negligence that no reasonable provider would have anticipated it, the chain of causation breaks.
Not every intervening event qualifies. If a surgeon’s error requires a second operation and the patient suffers a complication during that follow-up, the complication is a foreseeable consequence of the original error. Corrective procedures are a normal and expected response, so the second surgery doesn’t supersede the first provider’s negligence. But if a patient leaves against medical advice, seeks treatment from an unlicensed practitioner, and suffers a new injury from that practitioner’s incompetence, a court may view the patient’s choice as a superseding cause the original surgeon couldn’t have predicted. The more extraordinary and independent the intervening event, the more likely it qualifies.
Loss of Chance When the Patient Was Already Sick
Traditional causation rules create a harsh problem for patients who were already seriously ill. If you had a 40% chance of surviving cancer and the doctor’s delayed diagnosis reduced that to 10%, you can’t satisfy the standard “more likely than not” test. The delay didn’t tip the scales past 50% because you were already more likely to die than survive. Under traditional rules, you recover nothing despite losing a meaningful shot at survival.
The loss of chance doctrine addresses this gap. Roughly half of the states that have considered the issue have adopted some version of it, though the approaches vary. Some allow the jury to find full causation and award complete damages. Others treat the lost chance itself as the injury and award damages proportional to the percentage of survival probability the provider’s negligence destroyed.4PMC. Medicolegal Sidebar: The Law and Social Values: Loss of Chance A handful of states have explicitly rejected the doctrine, and at least three adopted it through their courts only to have the legislature revoke it later.
Whether you can bring a loss of chance claim depends entirely on the jurisdiction. In states that reject the doctrine, a patient whose pre-existing odds were below 50% has no causation theory available, no matter how badly the provider performed.
Expert Testimony Is How Causation Gets Proved
Causation in malpractice cases almost always requires expert testimony. A jury of non-physicians can’t evaluate whether a medication error caused organ damage or whether a delayed diagnosis changed a patient’s prognosis without someone qualified to explain the medicine.5PubMed Central. The Expert Witness in Medical Malpractice Litigation – Section: Introduction The expert reviews medical records, imaging, and lab results and offers an opinion connecting the provider’s conduct to the outcome.
That opinion must meet a specific threshold. Most jurisdictions require the expert to testify with “reasonable medical certainty” or “reasonable medical probability,” meaning the causal connection is a matter of professional conviction, not speculation.6National Conference of State Legislatures. Medical Liability/Malpractice Merit Affidavits and Expert Witnesses Saying “it’s possible the error caused the injury” isn’t enough. The expert needs to say it probably did.
These witnesses are expensive. Hourly rates for medical experts typically run from $325 to $450 for initial case review, and rates climb higher for depositions and trial appearances, where fees of $625 to $875 per hour are common depending on the specialty. Surgeons and neurologists tend to command the highest rates. Total expert costs in a single malpractice case can easily reach tens of thousands of dollars once you add up record review, report preparation, deposition, and trial testimony.
Getting the Opinion Admitted: Daubert and Frye
Not every expert opinion makes it in front of the jury. The trial judge acts as a gatekeeper. Federal courts and roughly two-thirds of states follow the standard set by Federal Rule of Evidence 702, which requires the proponent to show that the expert’s knowledge will help the jury, the testimony is based on sufficient facts, the methodology is reliable, and the expert applied that methodology properly to the case.7Legal Information Institute. Rule 702 – Testimony by Expert Witnesses This framework grew out of the Supreme Court’s 1993 decision in Daubert v. Merrell Dow Pharmaceuticals, which gave judges broad discretion to evaluate scientific evidence using factors like testability, peer review, error rates, and general acceptance.8National Institute of Justice. Daubert and Kumho Decisions
A smaller group of states still uses the older Frye standard, which asks only whether the expert’s methodology is “generally accepted” in the relevant scientific community. The practical difference matters. Under Frye, a novel but well-supported theory might get excluded if the field hasn’t broadly adopted it yet, while under Daubert the judge can evaluate the science on its own merits. Either way, weak expert testimony on causation gets challenged hard, and if the judge excludes it, the case typically collapses because the plaintiff has no other way to prove the medical link.
When Negligence Speaks for Itself
There’s a narrow exception to the expert testimony requirement. The doctrine of res ipsa loquitur (“the thing speaks for itself”) applies when the injury is the kind that simply doesn’t happen without negligence. A patient goes in for knee surgery and wakes up with nerve damage in the opposite arm. A surgical instrument gets left inside someone’s body. In these situations, you can invoke a presumption of negligence without hiring an expert to explain what went wrong.9National Center for Biotechnology Information. Medicolegal Sidebar: The Law and Social Values: Res Ipsa Loquitur
To use the doctrine, you generally must show three things: the injury is the type that doesn’t ordinarily happen without someone being negligent, the instrument or process that caused the injury was under the defendant’s exclusive control, and you didn’t contribute to the injury. When all three are met, the burden effectively shifts to the provider to explain what happened. Courts apply this exception sparingly, and it works best in cases where the negligence is so obvious that a layperson can see it without medical training.
The Burden Is a Preponderance of the Evidence
Medical malpractice is a civil case, so you carry the burden of proving causation by a preponderance of the evidence. That means demonstrating it’s more likely than not that the provider’s negligence caused the injury, essentially a greater than 50% probability.10Legal Information Institute. Preponderance of the Evidence This is a much lower bar than the “beyond a reasonable doubt” standard in criminal cases, but it’s still a real bar. If the evidence is evenly split, you lose.
In practice, meeting this standard requires building a coherent story from medical records, expert testimony, and the timeline of events that makes the jury believe the provider’s error probably produced the harm. The defense will offer its own experts and its own reading of the records. Close cases turn on the quality of expert testimony and whether the medical records support the plaintiff’s theory or undercut it.
How Patient Conduct Affects the Causation Analysis
The defense team will scrutinize everything you did before and after the alleged malpractice. If you contributed to your own injury, that conduct can reduce or eliminate the provider’s liability depending on the jurisdiction. Most states use a comparative negligence system that divides fault between the parties.11Legal Information Institute. Comparative Negligence
Under pure comparative negligence, a patient found 60% at fault can still recover 40% of damages. Under modified comparative negligence, which a majority of states follow, the patient is barred from recovering anything once their share of fault hits 50% or 51%, depending on the state. The behaviors that most commonly trigger comparative fault arguments include failing to follow a doctor’s instructions, delaying follow-up appointments, providing false or incomplete medical history, and attempting self-treatment before seeking professional care.12National Center for Biotechnology Information. Leading a Horse to Water AND Making Him Drink – Recommendations for Dealing with Non-Adherent Patients
There’s a limit, though. Courts are generally reluctant to blame patients for the condition that brought them to the doctor in the first place. A patient who arrives at the ER with a self-inflicted injury is still entitled to competent treatment. And if the doctor never properly explained what the patient was supposed to do, the patient’s “noncompliance” may not count against them at all, because the duty to inform rests on the provider.12National Center for Biotechnology Information. Leading a Horse to Water AND Making Him Drink – Recommendations for Dealing with Non-Adherent Patients In extreme cases, a patient’s intervening conduct can rise to the level of a superseding cause that breaks the chain of liability entirely, but courts set a high bar for that finding.
Procedural Hurdles That Can End the Case Before Causation Is Argued
Two procedural steps can kill a malpractice claim before the causation evidence ever gets heard, and both deserve attention early.
About 28 states require you to file an affidavit or certificate of merit early in the case.6National Conference of State Legislatures. Medical Liability/Malpractice Merit Affidavits and Expert Witnesses This is a sworn statement from a qualified medical expert confirming that, after reviewing the records, there is a reasonable basis to believe the provider fell below the standard of care and that the failure caused the injury. The expert who signs the affidavit typically practices in the same specialty as the provider being sued. Legislatures created this requirement to filter out cases that have no medical basis before they consume court resources. Missing the deadline or filing a deficient affidavit can result in dismissal with prejudice, meaning the case cannot be refiled.
Every state also imposes a statute of limitations on malpractice claims, and missing it destroys the case regardless of how strong the causation evidence is. The most common window is two years from the date of injury. Because many malpractice injuries aren’t immediately obvious — a misread pathology slide or a surgical error deep inside the body may not produce symptoms for months or years — most states apply a discovery rule that pauses the clock until you knew or reasonably should have known about the injury and its possible connection to negligence. States that apply the discovery rule typically also impose an outer deadline, often called a statute of repose, that cuts off claims after a fixed number of years no matter when the injury was discovered.