Medical Negligence Causation: Why the Chronology Decides It

In many clinical negligence claims, breach is the easier half, and medical negligence causation is the hard half. Showing that care fell below a reasonable standard means applying Bolam v Friern Hospital Management Committee [1957] 1 WLR 582, as qualified by Bolitho v City and Hackney HA [1998] AC 232: was the care supported by a responsible body of practitioners in the field, and does that body of opinion withstand logical analysis? The harder half, the half that decides the claim, is causation: proving the breach caused the harm. A claimant shows a clinician got it wrong and still loses, if they cannot show the error made the difference.

Causation turns on the medical facts, which means it turns on the expert’s evidence, which means it turns on the chronology. This piece sets out why causation is where claims are won and lost, how the legal tests work, and why a complete, sourced chronology is the foundation causation in clinical negligence stands on. It is written for medico-legal experts and the agencies that instruct them.

Why medical negligence causation is the harder half

Breach and causation are separate questions, and a claim needs both. Breach asks whether the care was negligent. Causation asks whether that negligence caused the injury. The two come apart more often than people expect. A missed diagnosis is a breach, but if the outcome would have been the same with a timely diagnosis, the breach caused no additional harm, and the claim fails on causation.

This is why causation is where the real contest happens. In many claims, the defence puts less weight on breach and concentrates on causation, arguing the outcome would have been the same regardless. That argument is won and lost on the medical facts: the patient’s condition before the breach, what the breach changed, and what followed. All of which live in the records, in sequence. The expert who commands that sequence controls the causation argument.

The but for test

The general rule for causation is the but for test. But for the breach, would the harm have occurred? If the answer is that the injury would have happened anyway, causation is not made out. If the answer is that the injury would have been avoided, causation is established. The test is applied on the balance of probabilities, meaning the claimant must show it is more likely than not.

Applying the test is a factual exercise before it is a legal one. To say what would have happened but for the breach, you have to establish what did happen, step by step: when the patient presented, what was recorded, what was done and not done, and how the condition changed over time. That is a chronology question. A but for opinion with a shaky grasp of the sequence is an opinion waiting to be dismantled in cross-examination.

When but for is not enough: material contribution

Some injuries do not resolve cleanly under the but for test, because more than one factor combined to cause them. Where a negligent cause and a non-negligent cause act together to produce a single, indivisible injury, and medical science cannot say whether the injury would have happened without the negligent one, the courts modify the test: material contribution.

The leading clinical negligence authority is Bailey v Ministry of Defence [2008] EWCA Civ 883, where the claimant’s injury resulted from a combination of negligent care and a non-negligent condition. The court held that causation was established because the negligence made a more than negligible contribution to the harm, even though it was not the sole cause. The principle traces back to Bonnington Castings Ltd v Wardlaw [1956] AC 613, and the Court of Appeal restated its application to indivisible injury in Holmes v Poeton Holdings Ltd [2023] EWCA Civ 1377. It was applied to a delay in treating sepsis in Williams v Bermuda Hospitals Board [2016] UKPC 4. Where causes combine to produce an indivisible injury, a more than negligible contribution from the breach is enough.

Three things are worth carrying. Most clinical negligence claims are still run on but for, so material contribution is the exception rather than a fallback to reach for whenever causes are mixed. The area remains unsettled: the Court of Appeal in Holmes called it bedevilled by inconsistency at the highest level, and further appellate consideration is widely expected. Where it does apply, establishing it demands an even clearer command of the sequence: which factors were in play, when each operated, and how they interacted. It also matters whether the injury is divisible or indivisible. Where an injury is divisible, its severity turning on how much of the causative agent was involved, the defendant answers only for the additional harm, and the expert may be asked to quantify the difference. Where it is indivisible, apportionment is impossible and the question becomes whether the breach contributed more than negligibly. That is the hardest kind of chronology to build and the most decisive when it is built well.

What the expert provides, and what the court decides

A line worth drawing clearly. The expert does not decide causation. The court applies the legal test. What the expert provides is the factual and medical foundation the test is applied to: what the records show, the sequence of events, and the medical opinion on whether and how the breach affected the outcome.

This keeps the expert in their lane and makes the evidence more useful, not less. The expert’s job is to give the court a clear, sourced account of the medical facts and a reasoned medical opinion on the effect of the breach. The court takes that and applies the but for or material contribution test. An expert who tries to reason in legal conclusions oversteps. An expert who lays out the medical sequence cleanly gives the court exactly what it needs to decide.

Why the chronology decides it

Every part of a causation opinion comes back to sequence. What was the patient’s condition before the breach. What did the breach change. What followed, and when. Causation is, at bottom, a question about the order of events and their consequences, and the answer lives in a chronology.

A causation opinion built on a complete, source-linked chronology is defensible, because every step in the reasoning traces to a record. When opposing counsel asks how the expert knows the condition was stable before the breach, the answer is a dated entry, not a recollection. A causation opinion built on a rushed or partial chronology is exposed, because the gaps are where the sequence breaks and the other side pushes. In a causation contest, command of the timeline is what the argument is fought on, because causation is the timeline.

Building the foundation causation needs

If causation rests on the chronology, the chronology has to be right: complete, in order, and traceable to source. That is difficult and slow to build by hand from a large, disordered bundle, which is where the sequence most often breaks down under time pressure. This is the foundation Health Narrator is built to produce.

It takes the full bundle and produces a structured, source-referenced chronology, every event in order and linked to the source line, shaped to the expert’s specialty. The expert sees the whole sequence the causation opinion depends on, checks any fact against its source in one click, and builds the opinion on ground that holds. The expert forms the causation opinion. The platform makes sure the sequence underneath it is complete and defensible. Users report up to 70 percent time saved on record review.

Causation is the harder half of most clinical negligence claims, and it is decided on the facts in the records, in order. Get the chronology right and the causation opinion has somewhere solid to stand. Get it wrong and no amount of clinical expertise saves it.

Build the causation opinion on a chronology that holds.

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Common questions about causation in clinical negligence

What is causation in clinical negligence?

Causation is the link between the breach of duty and the harm. Proving a clinician fell below the standard of care is not enough. The claimant must also show the breach caused the injury. In practice causation is often the harder half of the claim, and it turns on the medical facts, which is where the expert’s evidence and the chronology carry the weight.

What is the but for test in medical negligence?

The but for test is the general rule for causation: but for the breach, would the harm have occurred? If the injury would have happened anyway, causation fails. The test is applied on the balance of probabilities, meaning more likely than not. Establishing it depends on a clear account of what happened and when, which is what the medical chronology provides.

What is the material contribution test?

The material contribution test applies where medical science cannot say whether the injury would have happened but for the negligence, because several factors combined to cause an indivisible injury. Following Bailey v Ministry of Defence, causation is established if the breach made a more than negligible contribution to the harm, even alongside non-negligent causes. It is an exception rather than a general alternative to the but for test, and most claims are still run on but for.

How does a medical chronology support a causation opinion?

A medical chronology supports causation by fixing the sequence of events the opinion depends on. Causation is a question of what happened, in what order, and what followed. A complete, source-linked chronology lets the expert see where the breach sits in the timeline and what changed after it, and lets the opinion trace every step to the record. Without that sequence, a causation opinion rests on assertion.

Who decides causation in a clinical negligence case?

The court decides causation, applying the legal tests to the evidence. The medical expert does not decide the legal question. The expert provides the factual and medical foundation: what the records show, the sequence of events, and the medical opinion on whether and how the breach affected the outcome. The court then applies the but for or material contribution test to that evidence.