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A Guide to Causation - Part 1

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When investigating the merits of a prospective claim, a Clinical Negligence Lawyer has to establish if the Defendant was in breach of their duty of care to the Claimant (ie were they negligent) and if so, did that negligence cause or ‘materially contribute’ to an adverse outcome for the Claimant.

The ‘causation’ part of the investigation process is quite often more complicated than establishing if there was negligence.

If someone were to slip on a wet floor in a supermarket, fall and then fracture their wrist, it should be fairly easy to show that the slip caused the injury.

However, in most* Clinical Negligence cases, Claimants are already ill or injured before the alleged negligent act or omission, and therefore proving Causation is generally more complex and time consuming than in other types of personal injury cases.

* Exceptions include cases involving elective cosmetic surgery or family planning.


This is the simplest legal test for establishing causation. This involves asking the question :

“would the damage/injury have occurred but for the negligence of the defendant?”.

The test is not particularly scientific, as one simply has to show that on a balance of probabilities (ie that there was a 51% chance or more) the damage would not have been caused in the absence of the negligent act or omission.

As the test is so broad, it could be argued that in some cases a Defendant would be held liable for damages that he did not cause, and conversely there could be cases where a Defendant has escaped liability. This has led to a certain amount of criticism of the ‘but for’ test over the years.

Examples of the ‘but for’ test in action can be seen in the following cases:


In this case Mr Barnett was discharged from the Defendant Hospital, despite complaining of stomach pains. He died later that day from arsenic poisoning. Despite the Court finding that the Hospital was negligent for not treating Mr Barnett, they were ultimately found not to be liable for his death, as the evidence showed that even with treatment, he would still have died from the arsenic poisoning.

Therefore, when asking if Mr Barnett would have died of arsenic poisoning, but for the negligence of the Hospital, the answer is yes.


In this case the Claimant, who was a baby boy born prematurely, received excess oxygen – and as a result developed a condition know as RLF, rendering him almost blind. Breach of Duty was not disputed.

However, whilst expert medical evidence showed that excess oxygen can cause RLF, there is also a ‘causal link’ between RLF and at least four other conditions common in premature babies. The experts were unable to identify which of the potential causes was most likely to have caused the RLF. As such the case failed on causation, as the Claimant was unable to show that there was at least a 51% chance that the negligence (excess oxygen) led to the RLF.

Don’t miss Part 2 on Causation next week!

For more information or to speak to a member of the Clinical Negligence team click here