How to prove medical negligence: duty, breach, causation explained

Published 25 March 2026

TL;DR

To prove medical negligence you must establish three elements: duty of care, breach of that duty (assessed by the Bolam test), and causation (the breach caused the harm claimed)
All three elements must be proved on the balance of probabilities: it is more likely than not that each element is satisfied
Breach requires independent expert evidence from a clinician in the same specialism who confirms that the defendant's conduct fell below the standard no responsible body would have endorsed
Causation is usually the hardest element: you must prove that but for the breach, you would not have suffered the harm you are claiming for
Causation has specific rules in medical negligence that differ from general personal injury claims; loss of chance is not compensable following Gregg v Scott [2005]
Proving medical negligence in practice requires a solicitor to obtain records, instruct experts, and analyse causation before advising on the strength of a claim

Proving medical negligence is the central challenge in any clinical negligence claim. A bad outcome, a difficult experience, or dissatisfaction with treatment does not by itself amount to negligence. The law requires three distinct elements to be proved before any compensation can be recovered. Understanding what each element requires (and why each can fail independently) is essential for anyone considering whether they have a viable claim.

What are the three elements you must prove?

To prove medical negligence, you must establish all of the following on the balance of probabilities:

Duty of care: a healthcare professional owes a duty of care to their patient. This element is almost never in dispute in a clinical negligence claim. The duty arises automatically from the professional-patient relationship and exists whether the care is provided by the NHS or a private provider.

Breach of duty: the clinician must have acted in a way that fell below the standard of care required by law. This is assessed by the Bolam test. A clinician is not in breach if a responsible body of practitioners in the same specialism would have done the same thing in the same circumstances. Breach requires positive expert evidence: an independent clinician must confirm that what was done fell outside what any responsible body would have accepted.

Causation: the breach must have caused the harm claimed. This is assessed under the but-for test: would the harm have occurred even if the clinician had met the required standard? Causation must be proved to the same standard as breach: on the balance of probabilities.

How does the Bolam test work in practice?

The Bolam test for medical negligence originates from Bolam case [1957] 1 WLR 582. A clinician is not in breach of the standard of care if their conduct was accepted as proper by a responsible body of practitioners skilled in the relevant field. The test sets a floor: it does not ask whether the clinician made the best possible decision, but whether their decision fell within the range that any responsible body of peers would have endorsed.

The Bolitho refinement [1997] added an important qualification: the body of opinion relied upon by the defendant must be capable of withstanding logical analysis. Courts can reject expert opinion that, while sincerely held, is not logically defensible in light of the risks and benefits of the available options. In practice, courts rarely exercise this power, but it prevents the Bolam test operating as a blank endorsement of any opinion a colleague can be found to express.

To establish breach, your solicitor will instruct an independent expert in the relevant specialism. The expert will review the medical records and provide a report setting out whether the defendant's conduct fell below the standard and, if so, identifying the specific act or omission that constituted the breach.

Why is causation the hardest element to prove?

Causation in medical negligence is assessed under the but-for test. The question is: but for the clinician's breach, would the harm have occurred? If the answer is yes (the harm would have happened anyway even with proper treatment) there is no causation and no claim, regardless of how clear the breach was.

This creates particular difficulties in cases of delayed diagnosis. If a cancer would have been treated in the same way whether diagnosed six months earlier or at the actual date of diagnosis, the delay caused no compensable harm. If earlier diagnosis would have changed the treatment pathway or improved the prognosis, causation is established to the extent of that difference.

The House of Lords held in Gregg v Scott [2005] UKHL 2 that a reduction in the probability of a better outcome is not compensable. Loss of chance (losing a 40% prospect of recovery) does not give rise to a damages claim. You must prove that, on the balance of probabilities (more than 50%), the breach caused the specific harm claimed.

What evidence is needed to prove medical negligence?

The evidence required to prove medical negligence includes:

Medical records: the full clinical record is the foundation of any claim. This includes GP records, hospital notes, nursing records, imaging reports, laboratory results, operation notes, and correspondence between clinicians. Records are obtained by a subject access request and, in litigation, by formal disclosure.

Expert reports on breach: an independent clinical expert in the same specialism as the defendant reviews the records and gives an opinion on whether the standard of care was met. In complex cases, multiple experts may be needed: a GP expert for the referral failure and an oncology expert on what treatment should have been offered.

Expert reports on causation: a separate expert opinion on causation is often needed, addressing what would have happened if the correct standard of care had been applied. In a delayed cancer diagnosis claim, this will address staging, treatment options, and prognosis at the point when a timely diagnosis should have been made.

Quantum evidence: evidence of the financial losses flowing from the breach, including medical records showing the treatment received, employment records for lost earnings claims, and care assessments for care cost claims.

Balance of probabilities in medical negligence

The balance of probabilities standard means that something is more likely to have occurred than not: a probability greater than 50%. This applies to each of the three elements. You do not need to prove breach or causation to a criminal standard (beyond reasonable doubt). You need to prove that it is more likely than not that the defendant fell below the required standard and that this caused your harm.

In practice, this means that cases with strong expert support on breach but uncertain causation may not succeed. It also means that cases where the causation evidence is clear may succeed even where the breach evidence took time to establish. Both must be proved; proving one does not compensate for failing the other.

When breach is proved but causation fails

A common outcome in medical negligence claims is that independent expert evidence establishes a clear breach of the standard of care, but the causation analysis shows that the breach made no difference to the outcome. This is not negligence in the legally compensable sense. The claimant has no claim and is not entitled to compensation, regardless of the seriousness of the breach.

This is one reason why early expert review of both breach and causation is essential before significant costs are incurred. A solicitor specialising in clinical negligence will commission expert reports on both elements at the earliest practicable stage and advise on the strength of the claim as a whole.

For a full explanation of what constitutes medical negligence and the claim process from start to finish, see the relevant guides. For the step-by-step process, see the how to claim guide. For compensation ranges, see the compensation guide. For funding, see the funding guide.

This page provides legal information, not legal advice. If you believe you may have a medical negligence claim, speak to a qualified solicitor who specialises in clinical negligence.