Most arguments in a clinical negligence claim are not really about whether something went wrong. They are about whether the thing that went wrong made any difference. That second question is causation, and it is where many medico-legal cases are won or lost.
In short: causation asks whether the breach of duty actually caused the injury being claimed for. The starting point is the “but for” test: would the harm have happened but for the negligence? Where several causes act together and science cannot separate them, the courts may accept that the negligence made a material contribution. A lost chance of a better outcome is generally not compensated in clinical negligence. The expert’s job is to give an honest medical opinion on what probably happened, on the balance of probabilities, and to leave the legal conclusion to the court.
Prof. Sandip Hindocha, a Consultant Plastic Surgeon and the UK’s National Lead Forensic Medical Examiner, gives opinions on breach of duty and causation for claimant and defendant solicitors and as a single joint expert.
What Causation Means in a Negligence Claim
A claim in negligence needs three things to be proved: a duty of care, a breach of that duty, and loss caused by the breach. In clinical negligence the duty is rarely in dispute. Breach is judged against what a responsible body of practitioners would have done. Causation is the separate step that links the breach to the harm. For how that breach standard is defined, and the one area where a different test now applies, see our guide to how the test for clinical negligence changed from Bolam to Montgomery.
The two questions are easy to blur. An expert may conclude that the care was poor and that it caused no additional injury at all, and both halves of that opinion have to be reasoned separately.
The Standard of Proof Behind Every Causation Opinion
Causation is decided on the civil standard, the balance of probabilities, meaning more likely than not. How that standard works, and why it does not rise for more serious allegations, is covered in our guide to balance of probabilities in medico-legal opinions.
For the expert, this means a causation opinion should say which side of the line the evidence falls on, and why. “It is possible that” is not an opinion on causation. “On the balance of probabilities” followed by the clinical reasoning is.
The But For Test
The basic test asks whether the harm would have occurred but for the negligence. If the answer is yes, the harm would have happened anyway, causation fails.
The classic example is Barnett v Chelsea and Kensington Hospital Management Committee [1969] 1 QB 428. A man who had been poisoned with arsenic attended a casualty department and was sent home without being examined. He died. The hospital was found to have been in breach of duty, but the claim failed, because the evidence showed he would probably have died even with proper treatment. The failure to examine him did not cause his death.
Most causation disputes are a version of that question. Would earlier diagnosis have changed the outcome, or would the wound have healed with a better scar if the infection had been treated sooner? The sharpest version of it is a delay in diagnosing cauda equina syndrome, where the interval between presentation and decompression can be the only fact genuinely in dispute.
When the Breach Is a Failure to Act
The Bolitho Approach
Some claims turn on something a doctor did not do, such as failing to attend, refer or operate. Here causation has an extra layer. In Bolitho v City and Hackney Health Authority [1998] AC 232, the House of Lords considered a case where a doctor failed to attend a child in respiratory difficulty. The court had to ask what the doctor would have done had she attended, and, if she would not have intervened, whether failing to intervene would itself have been negligent.
What This Asks of the Expert
The expert therefore has to address the hypothetical. What would a competent clinician have done at each point, what would probably have followed, and how would the patient’s outcome have differed? That requires a clear timeline, judged without hindsight.
Material Contribution and Cumulative Causes
The but for test struggles where two or more causes operate together and the medical evidence cannot say which one tipped the balance. The law has developed the idea of material contribution for these cases.
In Bonnington Castings Ltd v Wardlaw [1956] AC 613, a worker developed lung disease from silica dust, some of which came from a source for which the employer was liable and some from a source for which it was not. The House of Lords held that it was enough that the negligent dust made a material contribution, meaning more than a negligible one, to the disease.
The principle was applied in a clinical setting in Bailey v Ministry of Defence [2008] EWCA Civ 883. A patient who had been left weak, partly by her illness and partly by negligent care, aspirated her vomit and suffered brain damage. The Court of Appeal accepted that the negligent care had materially contributed to the weakness that led to the injury. The Privy Council took a similar approach in Williams v Bermuda Hospitals Board [2016] UKPC 4, a case of delayed treatment of appendicitis where sepsis developed over time. The same cumulative pattern arises in delayed sepsis treatment claims, where organ dysfunction develops across the period of delay rather than at one identifiable point.
Material contribution is not a way around the but for test whenever proof is difficult. It applies where the causes act cumulatively and medical science cannot apportion them. Whether a case qualifies is a legal question, but it depends on how the expert describes the mechanism of injury.
Divisible and Indivisible Injuries
The distinction between divisible and indivisible injuries matters for both liability and damages. A divisible injury is one where the severity increases with exposure or with the extent of the negligent contribution, so the harm can be split. In Holtby v Brigham & Cowan (Hull) Ltd, decided by the Court of Appeal in 2000, damages for an industrial lung disease were reduced to reflect the part of the exposure for which the defendant was responsible. Apportionment of that kind divides responsibility between competing causes, which is a different exercise from how contributory negligence reduces damages for a claimant’s own conduct.
An indivisible injury cannot be meaningfully split. Many surgical complications fall into this category.
Scarring cases often raise the question in a practical form. Where a wound was caused by an accident and then repaired poorly, the expert may need to separate the scarring that the original injury would probably have left from any additional scarring, contracture or revision surgery attributable to the substandard repair. Our article on when a surgical scar becomes a negligence claim looks at how that assessment works for surgical scars.
Loss of Chance in Clinical Negligence
A claimant sometimes argues that the negligence did not cause the injury outright but reduced their chance of avoiding it. English law generally does not compensate that loss in personal injury and clinical negligence claims.
In Hotson v East Berkshire Area Health Authority [1987] AC 750, a boy’s hip injury was diagnosed late, and he developed a serious complication. The evidence was that he would probably have developed it anyway, even with prompt treatment, so the claim failed on causation. In Gregg v Scott [2005] UKHL 2, the House of Lords confirmed that a delay in diagnosing cancer, which reduced the claimant’s prospects of a cure from a chance that was already below 50 percent, did not give rise to a claim for the lost chance itself.
The expert evidence on prognosis therefore carries great weight, particularly where the likely outcome with proper treatment sits close to the 50 percent line.
Consent Cases and the Chester v Afshar Exception
Causation works differently where the complaint is a failure to warn of a risk. Normally the claimant has to show they would have made a different decision if properly warned. In Chester v Afshar [2004] UKHL 41, a patient was not warned of a small risk of nerve damage from spinal surgery, and the risk materialised. She could not show she would never have had the operation, only that she would not have had it on that day. The House of Lords nonetheless allowed the claim, treating it as a narrow modification of normal causation principles to give effect to the duty to warn.
In consent cases the expert is usually asked about the material risks and the reasonable alternatives. Whether the patient would have chosen differently is for the patient’s own evidence and the court.
Where Medical Science Cannot Give an Answer
Courts have been cautious about stretching causation where the science is uncertain. In Wilsher v Essex Area Health Authority [1988] AC 1074, a premature baby developed a sight-threatening condition that could have had several different causes, only one of which was linked to negligence. The House of Lords held that the claimant still had to prove, on the balance of probabilities, that the negligent cause was responsible.
The special rule in Fairchild v Glenhaven Funeral Services Ltd [2002] UKHL 22, which allows a claim where negligence materially increased the risk of disease, was developed for mesothelioma and has not been extended to clinical negligence generally. An expert should not frame a causation opinion as increased risk unless that is genuinely the most the evidence supports, and should say so plainly if it is.
The Expert’s Role in Causation
Medical Causation and Legal Causation
The expert gives an opinion on medical causation: what probably happened in the body and why. Whether that meets the legal test is for the court. A good causation section explains the mechanism of injury, the timeline, what different treatment would probably have achieved, and the evidence and literature behind each step.
Duties Under CPR Part 35
Under CPR Part 35, an expert’s overriding duty is to help the court on matters within their expertise, and that duty overrides any obligation to whoever instructed or pays them. Practice Direction 35 also requires the expert to summarise any range of opinion on an issue and give reasons for their own view. More on these duties is set out on our CPR Part 35 page.
Common Weaknesses in Causation Evidence
Causation opinions tend to be challenged for the same few reasons.
- Breach and causation are merged into one conclusion without separate reasoning.
- The opinion is expressed as a possibility rather than on the balance of probabilities.
- The report reasons backwards from the outcome, which invites criticism for hindsight.
- Pre-existing conditions and the natural course of the original injury are not addressed.
- The expert strays outside their specialty, for example commenting on a treatment decision in a field they do not practise.
Frequently Asked Questions
What is the but for test in medical negligence?
It asks whether the injury would have happened but for the negligent care. If the injury would probably have happened anyway, causation is not established, even where the care was substandard.
Can I claim for a lost chance of recovery?
Generally not in clinical negligence. Following Hotson and Gregg v Scott, a reduced chance of a better outcome is not compensated as a separate loss in personal injury claims in England and Wales.
Does the expert decide causation?
No. The expert gives an opinion on medical causation. The judge decides whether the legal test for causation is met, having heard all the evidence.
Why can a claim fail even when negligence is admitted?
Because breach and causation are separate. A defendant may admit substandard care but deny that it made any difference to the outcome, and the claim then turns entirely on causation.
Prof. Hindocha provides CPR Part 35 compliant opinions on breach of duty and causation in plastic surgery, scarring and medical negligence cases, for claimant and defendant solicitors and as a single joint expert. Details of how to instruct Prof. Hindocha are on the instruction page.
Related reading
- Balance of Probabilities, the Standard Behind Medico-Legal Opinions
- How to Write a High Quality Expert Witness Report
- When an Expert Witness Is Criticised by the Court
This article is general information about causation in civil and clinical negligence claims in England and Wales, and is not a substitute for legal advice on a specific claim.