What is Causation in Medical Negligence?

When bringing a medical negligence claim, it is not enough to show that a doctor or healthcare professional made a mistake. You must also prove that the error caused you an injury or harm.

This legal link between the breach of duty and the injury sustained is referred to as causation, and it is one of the most important, and often most complex, elements of a medical negligence claim.

Below, we explain how medical negligence causation works, the key legal tests used by the courts, and what claimants need to prove.

What is the "but for" test in medical negligence?

The “but for” test is the starting point for determining legal causation in medical negligence cases. It asks a simple question:

But for the doctor’s negligence, would the harm have occurred?

If the answer is no, then causation in medical negligence can be established. If the answer is yes, the claim may not be successful, even if the doctor clearly made a mistake.

For example, if a patient receives a delayed diagnosis or misdiagnosis and later suffers complications, the court will consider whether those complications would have happened anyway. If they would have occurred regardless of the error, it may not be possible to prove causation.

While the test for negligence causation sounds straightforward, in practice, it often requires detailed expert medical evidence to determine what would likely have happened if proper care had been given.

Can I sue if the doctor made a mistake, but I wasn’t harmed?

Generally, no. In medical negligence law, negligence without harm is not typically actionable.

A healthcare professional may have breached their duty of care, but if that breach did not cause injury, worsening of a condition, additional pain, or financial loss, it will be difficult to build a substantial medical negligence claim.

This can be understandably frustrating for patients who feel they have been let down by substandard care. However, compensation is designed to address actual loss or damage, not to punish mistakes alone. Professional regulators may still investigate errors, but civil claims require provable harm linked to the negligence.

What is material contribution in complex medical negligence cases?

In some medical negligence cases, the “but for” test is too simplistic, particularly where multiple factors contributed to the injury. This is where the concept of material contribution becomes relevant in medical negligence causation.

Instead of asking whether negligence was the sole cause of harm, the court considers whether the negligent act materially contributed to the injury. In other words, did it make a significant contribution, even if other causes were also involved?

Material contribution arguments often arise in cases involving:

Multiple negligent clinicians
Pre-existing medical conditions
Progressive illnesses
Cumulative injuries over time

If negligence can be shown to have materially worsened the patient’s outcome, causation may still be established.

How does loss of chance work?

Loss of Chance is a particularly complex area of medical negligence law.

In simple terms, it refers to situations where negligent treatment reduced a patient’s chance of recovery or survival. However, loss of chance claims are limited in medical negligence cases.

Generally, the courts require a claimant to prove that:

There was a greater than 50% chance of a better outcome, and
That chance was lost due to negligence

If the original chance of recovery was below 50%, a claim may not succeed, even if negligence reduced it further. This reflects the balance courts strike between probability and proof when handling causation in medical negligence.

What are intervening acts in a medical malpractice claim?

An intervening act (also known as a novus actus interveniens) is an event that occurs after the initial negligence and may break the chain of causation.

Examples might include:

Further negligent treatment by another clinician
An unrelated accident or injury
A patient failing to follow clear medical advice

If the intervening act is deemed significant enough, the original healthcare provider may no longer be legally responsible for the final outcome. Each case is highly fact-specific, and courts carefully assess whether the intervening act was foreseeable or truly independent.

Does an inherent risk count as negligence?

No. It is important to emphasise that not every poor outcome is caused by negligence.

Many medical treatments, such as surgery, carry inherent risks, even when performed correctly. If a complication was a known and unavoidable risk, and the patient was appropriately informed, it would not usually amount to negligence.

However, negligence may arise if:

Risks were not properly explained
Consent was inadequate
A risk was increased by negligent treatment

Distinguishing between unavoidable medical risk and negligent harm is a key part of proving causation.

What is proximate cause?

Proximate cause refers to how closely connected the negligence is to the harm suffered. The law does not require that the harm be the immediate result of the negligent act, but it must not be too remote.

In medical negligence claims, the court considers whether the injury was a reasonably foreseeable consequence of the breach of duty. If the harm is too distant or unexpected, causation may not be established.

How do you prove causation in a delayed diagnosis case?

Delayed diagnosis cases are among the most common and most complex medical negligence claims.

To prove causation, a claimant must usually show that:

The diagnosis should reasonably have been made earlier
Earlier diagnosis would have led to different treatment
That different treatment would have materially improved the outcome

This often involves detailed expert evidence comparing the patient’s actual outcome with the likely outcome had timely diagnosis occurred. The focus is not just on the delay itself, but on what difference that delay made.

Contact our medical negligence solicitors in Bristol

Causation is often the most challenging aspect of a medical negligence claim. Even where errors are clear, proving that they caused harm requires expert analysis and careful legal argument.

If you believe negligent medical care has caused you injury or worsened your condition, our experienced medical negligence solicitors can assess your case and explain your options clearly and honestly.

To book a free initial consultation with our medical negligence solicitors in Kingswood, Westbury-on-Trym or Chew Magna, please get in touch today.