The 3 Reasons Causation Is So Difficult to Prove in Medical Malpractice — And Why It Matters to Your Case

Of the four elements that must be established in a Canadian medical malpractice claim (duty of care, breach of the standard of care, harm, and causation), causation is consistently the most challenging to prove. It is the element most likely to determine whether an otherwise strong case succeeds or fails. And it is the element most frequently misunderstood by potential claimants who are trying to assess their own circumstances.

Understanding why causation is so difficult to establish, and what that difficulty means in practice, is essential for anyone considering a medical malpractice claim in Atlantic Canada. This post addresses the three core reasons that causation presents such a persistent challenge in this area of law.

how causation fits within the four elements of a medical malpractice claim

What Causation Requires in Canadian Medical Malpractice Law

Before examining why causation is difficult to establish, it is worth being precise about what establishing it actually requires. In Canadian civil litigation, causation must be proven on the balance of probabilities, meaning it is more likely than not that the breach of the standard of care caused or materially contributed to the harm suffered. This is a greater than fifty percent probability, not certainty.

The leading legal test for causation in Canadian negligence law is the "but for" test, established and refined through Supreme Court of Canada jurisprudence. Under this test, the claimant must demonstrate that but for the defendant's negligent act or omission, the harm would not have occurred. In straightforward negligence cases, this test is relatively manageable. In medical malpractice cases, it frequently is not, for three distinct and interconnected reasons.

Reason 1: Medicine Is Rarely an Exact Science

The first and most fundamental reason that causation is difficult to establish in medical malpractice cases is that medicine itself operates with a degree of inherent uncertainty that the law must grapple with, and does not always grapple with easily.

Unlike many other contexts in which negligence law operates, medical outcomes are not always predictable or traceable to a single cause. The human body is extraordinarily complex. Disease processes are not linear. Treatment responses vary significantly between individuals. Even in cases where a clinical error is clearly established, the question of whether a different course of action would have produced a different outcome is often genuinely uncertain, not because the evidence is poor, but because the underlying biology does not yield a definitive answer.

Consider a patient who presents with symptoms of a serious infection. The treating physician prescribes an antibiotic that turns out to be a suboptimal choice for the organism involved, and the patient subsequently develops serious complications. Was it the choice of antibiotic that caused those complications? Or would the infection have progressed to that point regardless of the antibiotic prescribed, given the virulence of the organism and the patient's individual physiology? The honest clinical answer, in many such cases, is that it is genuinely uncertain, and that uncertainty creates real difficulty in meeting the balance of probabilities standard that the law requires.

The absence of definitive published evidence linking a specific clinical decision to a specific outcome in a specific patient population is a recurring challenge in medical malpractice causation. Not every clinical question has been studied in a controlled trial. Not every drug interaction, treatment delay, or diagnostic error has a well-characterized body of literature quantifying its causal contribution to harm. Where that evidence does not exist, establishing causation requires clinical reasoning and inference, which is contestable, and which opposing experts will contest.

how the standard of care is established through expert evidence

Reason 2: Patients Are Medically Complex

The second reason causation is so difficult to establish is that the patients at the centre of medical malpractice claims are rarely uncomplicated. Most people who require medical care, particularly the kind of significant medical care that gives rise to potential malpractice claims, present with existing health conditions, risk factors, comorbidities, and vulnerabilities that independently affect their prognosis and their potential for adverse outcomes.

This complexity creates a fundamental challenge for causation. When a patient with multiple serious health conditions experiences a harmful outcome following an episode of substandard care, the question of what caused that outcome becomes genuinely multi-factorial. The harm may have been caused by the substandard care. It may have been caused by the progression of an underlying disease. It may have resulted from an interaction between the two. Or it may have been an outcome that was, to some degree, statistically likely regardless of the quality of care provided.

Disentangling these threads, isolating the contribution of the substandard care from the contribution of the patient's pre-existing conditions, is one of the most demanding tasks in medical malpractice litigation. It requires clinical expertise, a thorough review of the complete medical history, and often detailed engagement with the relevant medical literature. It is not a task that yields easy or intuitive answers, and it is rarely a task that produces certainty.

The Temporal Association Problem

One of the most common difficulties that arises from patient complexity is the tendency to interpret temporal association as causation. When a harmful outcome follows a medical event or decision in close proximity, it is natural and understandable for a patient to perceive the medical event as the cause of the harm. In some cases, that perception is correct. In others, it reflects the coincidence of timing rather than a true causal relationship.

The legal system requires more than temporal association to establish causation. A harm that follows a medical error must be shown to have been caused by that error, not merely to have occurred after it. That distinction, while conceptually clear, can be exceptionally difficult to establish in practice when the patient's underlying condition creates an independent pathway to the same harm.

how to assess whether your circumstances may support a viable claim

Reason 3: The Gap Between Scientific Standards and Legal Standards

The third reason causation presents such persistent difficulty in medical malpractice cases is a structural one, and it arises from the fundamental difference between how the scientific and medical communities think about causation, and how the legal system measures it.

Medical experts, the witnesses upon whom medical malpractice cases depend, are trained to think in terms of scientific certainty. Their instinct, when asked whether a particular clinical error caused a particular harm, is to reach for the level of confidence that the scientific method demands: a high degree of certainty, supported by robust evidence, before a causal conclusion is drawn. That instinct is entirely appropriate in a clinical or research context. It is, however, misaligned with the legal standard that applies in Canadian civil litigation.

The balance of probabilities requires only a greater than fifty percent likelihood: not scientific certainty, not consensus, and not proof beyond a reasonable doubt. An expert who would not say with scientific confidence that the error caused the harm may nevertheless be in a position to say, honestly and accurately, that it was more likely than not that the error caused the harm. Those are different conclusions, and the difference between them is the difference between a causation opinion that supports the claim and one that does not.

Navigating this gap, helping qualified experts understand the legal standard they are being asked to apply, and eliciting opinions that are both scientifically honest and legally meaningful, is one of the most nuanced skills in medical malpractice litigation. It requires a legal team that understands medicine well enough to engage with expert witnesses at a clinical level, and to frame the causation question in a way that the legal standard, rather than the scientific standard, is clearly understood and applied.

Why This Gap Matters for Claimants

For a potential claimant, the practical implication of this gap is significant. A case may have a credible and well-supported causation argument under the legal standard (a greater than fifty percent probability that the error caused the harm) while still being one that a medical expert, thinking instinctively in scientific terms, would be reluctant to endorse. Where that reluctance is rooted in a misunderstanding of the legal standard rather than a genuine absence of causal probability, it is a problem that informed and experienced legal representation can address. Where the reluctance reflects an honest assessment that the causal probability does not reach fifty percent, it is a signal about the viability of the claim itself.

Distinguishing between these two situations requires both clinical knowledge and experience with how causation is assessed in the relevant courts, precisely the combination of expertise that Acuity Medical Law brings to every case it evaluates.

how Acuity Medical Law's clinical expertise informs its approach to causation

What Strong Causation Evidence Looks Like

Given how difficult causation is to establish, it is worth understanding what strong causation evidence actually looks like in a medical malpractice case. While no two cases are identical, the following factors tend to support a strong causation argument.

First, a clear and well-documented clinical mechanism connecting the error to the harm: a biologically coherent pathway that explains, step by step, how the substandard care produced the outcome that occurred. Second, published medical literature supporting that mechanism: studies, guidelines, or case series that establish the known relationship between the type of error and the type of harm. Third, a patient presentation and medical history that do not offer compelling alternative explanations for the harm: cases where the pre-existing complexity is limited, or where the pre-existing conditions are clearly insufficient to account for the outcome on their own. And fourth, expert witnesses who can engage confidently and clearly with the balance of probabilities standard and provide opinions that are both clinically sound and legally meaningful.

Where these factors are present, causation can be established convincingly. Where they are absent or weak, the causation question becomes the central battleground of the litigation, and often the reason a case that appears strong on the standard of care does not ultimately succeed.

what makes a medical malpractice case strong overall

The Importance of Early Causation Assessment

Because causation is so frequently determinative of whether a medical malpractice case can succeed, it is essential that it be assessed rigorously and honestly at the earliest possible stage. A case that is strong on duty of care and breach, but weak on causation, may not be viable, and the earlier that assessment is made, the better positioned the potential claimant is to make an informed decision about whether to proceed.

Acuity Medical Law's approach to preliminary case assessment includes a careful and clinically informed evaluation of causation from the outset. The firm's in-house medical expertise allows for an independent assessment of the causal question before formal expert reports are commissioned, providing a realistic and grounded view of whether causation is likely to be establishable on the facts of the particular case.

what the initial case assessment process at Acuity Medical Law involves