In clinical negligence cases, the issue of causation is often complex and the most difficult legal test for the Claimant to prove. There have been a large number of  cases where the court has considered the issue of causation in great detail and the recent case of ST (A protected party by his Mother and Litigation Friend KT) v Maidstone & Tunbridge Wells NHS Trust is no exception.

In clinical negligence claims the Claimant must prove that on the balance of probabilities there is a 51% chance a past event occurred or would have occurred. However, in some instances the injury may have had several causes and it may not therefore be possible to satisfy the ‘but for’ test. This was considered at length in the case of Bailey v Ministry of Defence (2009).

Bailey v Ministry of Defence (2009)

In the case of Bailey the Claimant aspirated vomit leading  to a cardiac arrest and hypoxic brain damage. The Claimant was treated negligently but he also had pancreatitis.

In Bailey, it considered those cases where the injury could have been caused by both negligent and non-negligent acts or omissions. In these circumstances, it was ruled that the ‘but for’ test for causation should be relaxed i.e. the defendant may be liable where its negligence made a material contribution to the injury.

In this case it was proven that medical science would be unable to put a percentage value on how much each of the acts contributed to the injury. Therefore if the Claimant can show the breach was “more than negligible” in contributing to the injury, then the test would be satisfied.

In Bailey the Court of Appeal found that both pancreatitis and the negligence had an impact and therefore the evidence did not satisfy the ‘but for’ test.

The Court of Appeal went on to set out the key elements concerning ‘material contribution’ in causation:

  1. A number of factors caused the injury
  2. The claimant is unable to satisfy the ‘but for’ test due to lack of evidence or medical science cannot prove it
  3. The Defendant is unable to show that ‘but for’ the negligence the damage would have been the same
  4. The claimant can prove that the negligence was ‘more than negligible’ or that it made a ‘material contribution’ to the severity or extent of the injury.
  5. The extent or percentage of the negligence cannot be measured

ST v Maidstone & Tunbridge Wells NHS Trust (2015) Bailey was considered at length in ST v Maidstone & Tunbridge Wells NHS Trust.

The Claimant suffered congenital haematological conditions including hereditary spherocytosis. The level of haemoglobin (Hb) in the blood for an average child is 11. ST’s level was much lower.

When his Hb level dropped below 5 he required an emergency blood transfusion. When ST was 2½ years  old he developed an upper respiratory tract infection. On 24th November 2004 his Hb level was 3.3. The hospital discharged ST home and advised that he would need to return the next day for a transfusion.

At 11.00 on 25th November 2004 ST underwent a one hour transfusion.

On 26th November 2004, ST required a four hour transfusion. Shortly after, ST’s condition deteriorated and he suffered seizures and a stroke resulting in permanent brain damage.

The Defendant Trust admitted that the transfusion should have commenced by 02.00 on 25th November 2004. The Court also found that the failure to properly administer IV fluids and the decision to use Frusemide during both transfusions amounted to negligence.

The Court was also satisfied that on the balance of probabilities the primary cause of the claimant’s strokes was a focal cerebral arteriopathy caused by the upper respiratory tract infection. 

In the Claimant’s first argument they alleged that the failure of the Trust to provide a transfusion earlier caused there to be a risk (albeit a small risk) of ST suffering with cardiac problems leading to a brain injury. However, the Claimant failed to satisfy the strict ‘but for’ test.

For the Claimant’s second argument they referred to the case of Bailey. They alleged that the Defendant Trust’s failures ‘materially contributed’ to ST’s injuries.

Swift J concluded on this exact point at paragraph 215 of his judgment:

‘… the Claimant faces similar problems to those which I referred to when dealing with his primary case. Once again, I accept that there is a possibility that the various conditions, or some of them, may have combined cumulatively or synergistically with the arteriopathy to cause his strokes. However, there is no objective and reliable evidence that the Claimant’s dehydration, acute chronic haemolysis and/or severe anaemia, whether together or separately, contributed with the arteriopathy to cause his strokes. That being the case, I cannot reach the conclusion that it was so and the Claimant’s secondary case must fail’.


Claimants mostly fall short on their causation evidence being unable to establish the ‘but for’ test. As a result, judgments reporting specifically on causation are lengthy and complex.

In the case of ST v Maidstone & Tunbridge Wells NHS Trust, the case details are also complex and this makes the analysis  of the causation principle even more difficult. However, the case was decided upon based on its own facts. In summary, the result in ST showed that it can be extremely difficult to prove that the Defendant’s failures materially contributed to the cause of the Claimant’s injuries.

No doubt this will not be the last time we hear about this causation principle and there will again be an in-depth judgment on material contribution causation in the near future. For the moment though, the case of ST confirms the legal test set out in Bailey.