Establishing that a healthcare professional has breached their duty of care does not automatically mean they are legally responsible for the injury suffered by a patient. There is one essential question to answer first: did that breach cause the harm?
In clinical negligence claims, this is examined using what is known as the 'But For' test. It is a fundamental principle of factual causation in English negligence law and asks a seemingly straightforward question: but for the negligent act or omission, would the patient have suffered the injury in question? The question sounds simple, but applying it to clinical evidence can be complex.
What does the 'But For' test mean?
The test considers what would probably have happened if the healthcare professional had provided the appropriate standard of care. In other words, it requires an examination of an alternative scenario in which the alleged negligence did not occur. Would the patient have avoided the injury? Would the outcome have been different? Or would the same harm have occurred regardless of the breach?
For a claimant to establish factual causation under the ordinary 'But For' test, they must demonstrate that, on the balance of probabilities, the injury would not have occurred without the negligent act or omission.
The balance of probabilities means that something is more likely than not to have happened. It is not necessary to establish causation beyond all doubt, but the evidence must support the conclusion that the negligence probably made the difference.
When a breach does not establish causation
An example of this principle can be found in the case of Barnett v Chelsea and Kensington Hospital Management Committee [1969]. Although this case does not concern non-surgical aesthetic treatments, which are the main focus of my medico-legal work, it provides a useful illustration of how the 'But For' test is applied in clinical negligence claims.
Three men attended a hospital complaining of vomiting after drinking tea. The casualty officer advised them to go home and contact their doctors. One of the men subsequently died from arsenic poisoning. The hospital's failure to examine the patient was found to be negligent. However, the court also considered whether an examination and appropriate treatment would have prevented his death.
The medical evidence established that, even if he had been admitted and treated, his condition was already so severe that he would probably still have died. Consequently, although there had been a breach of duty, the claimant could not establish that it had caused the death. The injury would probably have occurred anyway.
The case demonstrates that negligence and causation are separate questions. A healthcare professional may have acted below the required standard, but unless the claimant can establish the necessary causal connection between that breach and the harm, a claim for that injury will not succeed.
Applying the test to clinical practice
Consider a hypothetical example involving a delayed diagnosis.
A patient presents with symptoms that should have prompted more investigation. The practitioner fails to arrange the appropriate tests, and the patient is diagnosed with a serious condition several weeks later.
There may be evidence that the failure to investigate fell below the expected standard of care. However, establishing that breach is only the beginning of the causation analysis. The next question is what would probably have happened if the investigations had been arranged at the appropriate time.
Would the condition have been diagnosed earlier? Would treatment have been available that would probably have prevented the subsequent injury or reduced its severity? Or was the condition already at a stage where the eventual outcome would have been the same, even with appropriate care?
The answers depend on the medical evidence, including the nature and progression of the condition, the available treatment options and the likely outcome at the relevant time. It is not enough to demonstrate that an earlier diagnosis would have been preferable. The claimant must establish that the delay made a difference to the injury for which compensation is sought.
Why medical evidence is so important
The 'But For' test requires experts to consider a counterfactual clinical scenario. This means examining what would probably have happened if the negligent act or omission had not taken place.
For an expert witness, this involves an objective assessment of the available evidence, the patient's clinical history, the relevant medical science and the likely consequences of alternative courses of action. This can be challenging where a patient has pre-existing conditions, multiple risk factors or an underlying disease that could independently have contributed to the outcome.
A temporal relationship between an act and an injury does not, by itself, establish causation. The fact that a complication developed after a particular treatment does not necessarily mean the treatment caused it. Equally, the existence of another possible cause does not automatically exclude negligence as a contributing factor. The task is to assess the competing explanations and determine what the evidence supports on the balance of probabilities.
Is the 'But For' test always applied?
Although the 'But For' test is the usual starting point for establishing factual causation, it is important to recognise that it does not resolve every case.
There are circumstances where the relationship between negligence and injury is more complex, particularly where several factors have contributed to the same injury and medical evidence cannot reliably separate their respective contributions.
The Court of Appeal considered this in Bailey v Ministry of Defence [2008]. The case involved a patient whose weakness had resulted from both negligent and non-negligent causes. The court accepted that, in certain circumstances involving cumulative causes, a claimant may establish causation by demonstrating that the negligent contribution was material, rather than having to satisfy the conventional 'But For' test.
This is not a general alternative available whenever causation is difficult to prove. Its application depends on the particular facts and the nature of the medical evidence.
Establishing the link between negligence and harm
The 'But For' test is a reminder that clinical negligence claims require more than identifying an error or an adverse outcome. They require evidence connecting the two. For expert witnesses, this means maintaining a clear distinction between what is known, what can reasonably be inferred from the clinical evidence and what remains uncertain.
An adverse outcome does not automatically establish negligence, just as an established breach does not automatically prove that the breach caused the injury. Ultimately, the question is whether the evidence demonstrates that the patient's outcome would probably have been different if appropriate care had been provided.