Introduction
In any claim for negligence, a claimant must establish that the defendant owed them a duty of care, that this duty was breached, and that the breach caused the claimant to suffer legally recognised damage. Causation is a fundamental element, acting as the necessary link between the defendant’s wrongful conduct and the claimant’s harm. The issue of causation is divided into two parts: factual causation and legal causation. Factual causation asks whether the defendant’s breach was, as a matter of fact, a cause of the harm. Legal causation considers whether the harm was too remote a consequence of the breach for the defendant to be held legally responsible. This essay will focus on the specific challenges of establishing factual causation in medical negligence cases.
The starting point for determining factual causation is the 'but for' test. However, the complexities of medical science often make this test difficult to satisfy, potentially leading to injustice for claimants. In response, the courts have developed limited exceptions to the standard test. This essay will explain the operation of the 'but for' test using key case law before examining the judicial development of the 'material contribution to injury' and 'material contribution to risk' doctrines. It will be argued that while the courts have shown some flexibility, they have been cautious in departing from the traditional test, especially in the medical context, creating a body of law that attempts to balance certainty with fairness.
The 'But For' Test: The Orthodox Approach
The primary test for factual causation is the 'but for' test. This asks the question: 'but for the defendant’s negligent act or omission, would the claimant have suffered the harm?' If the harm would have occurred anyway, regardless of the defendant’s conduct, then causation is not established, and the claim will fail. The claimant bears the burden of proving, on the balance of probabilities (i.e., that it is more than 50% likely), that the breach was a necessary cause of their injury (Horsey and Rackley, 2021).
The classic illustration of this principle is found in Barnett v Chelsea & Kensington Hospital Management Committee [1969] 1 QB 428. In this case, a man presented at a hospital emergency department complaining of vomiting after drinking tea. The doctor on duty, who was also unwell, failed to examine him and sent him home, advising him to see his own GP. The man died a few hours later from arsenic poisoning. His widow sued the hospital. The court found that the hospital had breached its duty of care by failing to examine and treat the man. However, the claim failed on causation. The medical evidence showed that even if the man had been admitted and treated with all due care, he would have died anyway because the poison was already too advanced in his system. Therefore, the doctor’s negligence was not a 'but for' cause of the death. The harm would have occurred regardless.
The 'but for' test provides a clear and logical starting point. It ensures that a defendant is only held liable for consequences they have actually caused. However, in complex medical cases where a patient’s condition may have multiple potential causes, or where scientific knowledge is incomplete, rigidly applying this test can make it almost impossible for a claimant to succeed.
Material Contribution to Injury
The first significant modification to the 'but for' test is the 'material contribution to injury' doctrine. This applies in situations involving 'cumulative' causes, where the defendant's breach combines with other factors (which may be non-negligent or 'innocent') to produce the final injury. In such cases, the claimant does not have to prove that the breach was the sole or even the main cause of the harm. Instead, they only need to show that the defendant’s negligence made a 'material' contribution to the injury. 'Material' in this context means a contribution that is more than negligible (Mulheron, 2020).
The principle was established in the industrial disease case of Bonnington Castings Ltd v Wardlaw [1956] AC 613. A factory worker contracted pneumoconiosis from inhaling silica dust. Some of the dust exposure came from a source for which the employer was not negligent (the 'innocent dust'), while some came from a source for which the employer was negligent (the 'guilty dust'). It was impossible to determine which source of dust had actually caused the disease, as the condition was the result of a cumulative build-up. The House of Lords held that the claimant only had to prove that the guilty dust had materially contributed to the disease. As it had, the defendant was liable for the whole of the harm.
This principle was directly applied to a medical negligence scenario in Bailey v Ministry of Defence [2008] EWCA Civ 883. The claimant suffered brain damage following a cardiac arrest. The arrest was caused by her physical weakness, which in turn was caused by two factors: her underlying (non-negligent) pancreatitis, and the negligent lack of care she received from the defendant hospital post-operation. It was not possible to say whether she would have avoided the injury 'but for' the negligent care. The Court of Appeal held that the Bonnington principle could be applied. As the negligent care had materially contributed to her state of weakness, and the weakness caused the cardiac arrest, causation was established. This case shows the courts’ willingness to use the material contribution doctrine to overcome evidential gaps in medical cases where multiple factors have clearly contributed to the final outcome.
Material Increase in the Risk of Injury
A more radical and controversial departure from the 'but for' test is the concept of 'material increase in the risk' of injury. This exception applies in very specific circumstances where it is scientifically impossible to prove which of several potential sources caused the harm, but it can be shown that the defendant’s breach materially increased the risk of the claimant contracting the disease.
The origin of this exception is McGhee v National Coal Board [1973] 1 WLR 1, where a worker contracted dermatitis. The cause was brick dust. Some exposure during the working day was non-negligent, but the employer was negligent in failing to provide showers, meaning the claimant had to cycle home with the dust still on his skin. It was impossible to prove whether the dermatitis was caused by the 'innocent' exposure during the day or the 'guilty' extended exposure after work. The House of Lords held that by increasing the duration of the exposure, the defendant had materially increased the risk of the claimant contracting the disease, and this was sufficient to establish causation. This was confirmed in Fairchild v Glenhaven Funeral Services Ltd [2002] UKHL 22, a case concerning multiple employers who had exposed the claimants to asbestos, leading to mesothelioma.
However, the courts have been extremely reluctant to apply this principle in the context of medical negligence. The key case is Wilsher v Essex Area Health Authority [1988] AC 1074. A premature baby was given excess oxygen due to a monitoring error by the hospital. The baby later went blind from a condition called retrolental fibroplasia (RLF). The excess oxygen was one of five possible causes of the RLF; the other four were natural consequences of premature birth. The claimant could not prove on the balance of probabilities that the excess oxygen was the cause. The House of Lords refused to apply the McGhee exception. They distinguished McGhee on the grounds that in Wilsher there were multiple different and distinct risk factors at play (e.g., hypercarbia, intraventricular haemorrhage, etc.), whereas in McGhee there was only one causal agent (brick dust). The court decided that to allow the claim would be to reverse the burden of proof and effectively make the hospital liable simply because they were one of several possible causes.
This decision significantly limits the application of the material increase in risk doctrine in medical cases, confirming that the 'but for' test remains the standard. It highlights a judicial concern that a broader application would open the floodgates to claims where medical science cannot provide a definitive answer on causation.
The Rejection of 'Loss of a Chance'
A further attempt to circumvent the strictness of the 'but for' test has been through arguments based on the 'loss of a chance' of a better outcome. Claimants have argued that even if they cannot prove the negligence caused the final harm, they should be compensated for the lost chance of recovery. The courts have consistently rejected this approach in medical negligence cases concerning physical injury.
In Hotson v East Berkshire Area Health Authority [1987] AC 750, a boy injured his hip in a fall. The hospital negligently failed to diagnose his injury for five days. Medical evidence stated that there was a 75% probability that the severe injury he ultimately suffered would have developed even with prompt treatment. The trial judge awarded him 25% of the full damages, representing the lost chance of a full recovery. The House of Lords overturned this. They held that the question had to be decided on the balance of probabilities. Since it was 75% likely that the damage was already inevitable when the boy arrived at the hospital, the claimant had failed to prove that the hospital's negligence was a 'but for' cause of his injury.
This position was confirmed in Gregg v Scott [2005] UKHL 2, where a doctor negligently misdiagnosed a lump under the claimant’s arm as benign when it was cancerous. This led to a nine-month delay in treatment, during which the cancer spread. The delay reduced the claimant's chance of surviving for ten years from 42% to 25%. A narrow majority in the House of Lords rejected his claim for the reduction in his survival prospects. Because his initial chance of survival was already below 50%, he could not prove that 'but for' the negligence, he would probably have been cured. The majority expressed fears that allowing such claims would lead to liability in almost every case and undermine the 'but for' test.
Conclusion
In conclusion, the law on factual causation in medical negligence cases is dominated by the 'but for' test. This provides a clear, albeit sometimes harsh, standard for claimants to meet. The courts have recognised the potential for injustice where the standard test is impossible to satisfy due to evidential uncertainty, particularly where multiple factors contribute to an injury. This has led to the development of the 'material contribution to injury' doctrine, as seen in Bailey, which offers a path to justice in cases of cumulative causes.
However, the judiciary has shown a marked resistance to extending the exceptions further. The refusal to apply the 'material contribution to risk' doctrine in Wilsher and the rejection of 'loss of a chance' claims in Hotson and Gregg demonstrate a clear policy of confining departures from the 'but for' test to very narrow circumstances. The law, therefore, remains in a state of tension. It balances the need for a claimant to prove their case against the understanding that medical science is not always certain. While this leads to some difficult outcomes, the courts have prioritised certainty and the integrity of the 'but for' principle over creating wider exceptions that could significantly expand the scope of liability for medical professionals and the NHS.
References
Bailey v Ministry of Defence [2008] EWCA Civ 883.
Barnett v Chelsea & Kensington Hospital Management Committee [1969] 1 QB 428.
Bonnington Castings Ltd v Wardlaw [1956] AC 613.
Fairchild v Glenhaven Funeral Services Ltd [2002] UKHL 22.
Gregg v Scott [2005] UKHL 2.
Horsey, K. and Rackley, E. (2021) Tort Law. 7th edn. Oxford: Oxford University Press.
Hotson v East Berkshire Area Health Authority [1987] AC 750.
McGhee v National Coal Board [1973] 1 WLR 1.
Mulheron, R. (2020) Principles of Tort Law. 2nd edn. Cambridge: Cambridge University Press.
Wilsher v Essex Area Health Authority [1988] AC 1074.
