Few phrases cause more confusion for new expert witnesses than “but for” causation.
The phrase sounds technical, but the underlying question is simple. It asks:
If the negligent act or omission had not happened, would the patient probably still have suffered the same outcome?
That is the essence of “but for” causation.
In clinical negligence cases, breach of duty and causation are separate questions. A clinician may have made an error, but that does not necessarily mean the error caused the injury. Equally, a patient may have suffered a serious adverse outcome, but that does not necessarily mean the outcome was caused by negligence.
The “but for” question is designed to test the causal link between the breach and the outcome. It asks the expert witness to compare two pathways:
- The actual pathway: what really happened.
- The counterfactual pathway: what probably would have happened if reasonable, non-negligent care had been provided.
The Court is not asking whether a better outcome was possible. It is asking whether a better outcome was probable.
That distinction is crucial.
What Does “But For” Actually Mean?
The words “but for” can be misleading because they are not used in ordinary clinical language. Doctors, dentists, nurses and other clinicians do not usually say, “but for the failure to prescribe antibiotics, the patient would have recovered.” They say something more natural, such as:
- “If antibiotics had been started earlier, the patient would probably have avoided sepsis.”
- “If the scan had been performed, the diagnosis would probably have been made.”
- “If the patient had been referred urgently, treatment would probably have started sooner.”
- “If the tooth had been monitored properly, it probably would not have been lost.”
- “If the retinal detachment had been diagnosed earlier, the visual outcome would probably have been better.”
Those are all “but for” opinions.
The phrase can therefore be translated as:
“If not for the breach, what probably would have happened?”
Or, even more simply:
“Take the negligence away. Does the injury still happen?”
If the injury would probably still have happened, factual causation is usually not established. If the injury would probably have been avoided, or materially less severe, factual causation may be established.
A Simple Example
Consider a patient who attends hospital with symptoms of infection. There is a negligent six-hour delay in administering antibiotics. The patient develops septic shock.
The breach question is:
Should antibiotics have been given earlier?
The causation question is different:
If antibiotics had been given earlier, would septic shock probably have been avoided?
The answer might be yes. The answer might be no. It depends on the clinical facts, including the patient’s condition at presentation, observations, blood tests, source of infection, organism, co-morbidities, response to treatment and timing of deterioration.
The expert must not assume causation simply because there was a delay. The expert must explain what would probably have happened in the alternative scenario.
The “But For” Test as a Two-Column Exercise
One of the simplest ways for expert witnesses to understand “but for” causation is to imagine two columns.
| Actual events | Counterfactual events |
|---|---|
| What actually happened? | What probably would have happened with reasonable care? |
| What was missed, delayed or done wrongly? | What should have been done instead? |
| What injury or deterioration occurred? | Would that injury or deterioration probably have been avoided? |
This two-column approach is especially useful in report writing. It prevents the expert from jumping directly from breach to outcome.
For example:
| Actual pathway | Counterfactual pathway |
|---|---|
| Patient was not reviewed at 02:00. | Patient should have been reviewed at 02:00. |
| Deterioration continued overnight. | Review would probably have identified hypotension, tachycardia and hypoxia. |
| No antibiotics or escalation occurred until morning. | Antibiotics, fluids and senior escalation would probably have occurred overnight. |
| Patient suffered cardiac arrest at 07:00. | On the balance of probabilities, earlier treatment would probably have avoided the arrest. |
This is the practical meaning of “but for”. The expert is being asked to construct the likely non-negligent pathway and compare it with what actually happened.
“Would Have”, Not Merely “Could Have”
Another common source of confusion is the difference between possibility and probability.
In clinical practice, it is natural to say that earlier treatment “could have helped” or “might have made a difference”. That may be clinically true, but it is usually not enough for legal causation.
The Court generally needs an opinion on what would probably have happened.
Weak causation language includes:
- “Earlier treatment could have improved the outcome.”
- “The delay may have contributed.”
- “It is possible that the injury would have been avoided.”
- “The patient might have done better.”
Stronger causation language includes:
- “On the balance of probabilities, earlier treatment would have avoided the injury.”
- “It is more likely than not that the same outcome would have occurred in any event.”
- “In my opinion, the delay materially contributed to the deterioration.”
- “The breach probably made no difference to the final outcome.”
This does not mean that the expert must pretend to be certain. The standard is not certainty. The standard is probability. If the evidence does not permit a probable conclusion, the expert should say so.
Why “But For” Is Not the Same as “Caused By”
Experts sometimes hear “but for causation” and assume it simply means identifying what caused the injury. That is not quite right.
In medicine, many things may be causally relevant. A patient may deteriorate because of infection, co-morbidity, frailty, delay, inadequate monitoring, late antibiotics and poor physiological reserve. All may form part of the clinical story.
The legal question is narrower:
Did the breach make a legally relevant difference to the outcome?
For example, a patient with advanced cancer may experience a delayed diagnosis. The delay is a breach. The delay may also be emotionally distressing. However, if the cancer was already incurable and earlier diagnosis would not have changed treatment, survival or prognosis, “but for” causation for the major injury may fail.
By contrast, if earlier diagnosis would probably have allowed curative treatment, less extensive surgery, better survival or avoidance of metastasis, causation may be established.
The expert therefore needs to avoid vague statements such as:
The delay caused a worse outcome.
Instead, the expert should say:
Had diagnosis occurred in January rather than June, the tumour would probably have been stage II rather than stage III. On the balance of probabilities, the patient would have undergone less extensive treatment and would have had a materially better prognosis.
That is much more useful.
“Take the Breach Away”
A practical way to teach “but for” causation is this:
Take the breach away and ask what remains.
If the same outcome probably remains, causation is not made out.
If the outcome probably changes, causation may be made out.
Examples:
Example 1: No causation
A dentist fails to diagnose advanced periodontal disease for three months. The patient later loses several teeth. The records show severe bone loss had been present for many years and the teeth were already hopeless at the time of the alleged delay.
The breach may be established. However, if the teeth would probably have been lost even with non-negligent care, the three-month delay did not cause the tooth loss.
In “but for” language:
But for the three-month delay, the teeth would probably still have been lost.
Example 2: Causation established
A patient reports flashes, floaters and a shadow in vision after eye surgery. There is a negligent delay in referral. Retinal detachment is diagnosed several days later with macular involvement and poor visual outcome.
If earlier review would probably have diagnosed the detachment before macular involvement, and earlier surgery would probably have preserved better vision, causation may be established.
In “but for” language:
But for the negligent delay, the patient would probably have undergone earlier surgery and avoided the same degree of visual loss.
Example 3: Causation uncertain
A patient has delayed antibiotics in the context of severe sepsis and multiple co-morbidities. The evidence shows that earlier antibiotics might have improved the outcome, but it is not possible to say that survival was more likely than not.
In “but for” language:
It is possible that earlier antibiotics would have changed the outcome, but I cannot say that the patient would probably have survived.
That may be an uncomfortable conclusion, but it is sometimes the correct expert opinion.
The Core Question for the Expert Witness
The expert should repeatedly return to this question:
What probably would have happened with reasonable care?
That is the heart of “but for” causation.
A useful report-writing formula is:
In the actual course of events, [describe what happened].
In my opinion, reasonable care required [describe non-negligent care].
Had that occurred, [describe what would probably have happened].
Therefore, on the balance of probabilities, [the injury would / would not] have been avoided or materially reduced.
This is much clearer than using the words “but for” repeatedly. In fact, expert witnesses do not need to overuse the phrase. What matters is that they answer the question behind it.
How Bolitho Helps Explain “But For”
Bolitho is a useful teaching case because it shows that, in an omission case, it is not enough to say that someone should have attended. The expert must ask what would have happened if they had attended.
The case concerned a child who suffered respiratory difficulties in hospital. A doctor failed to attend. The claimant argued that, had the doctor attended, the child would have been intubated and the subsequent injury avoided.
The causation question was not simply:
Was it negligent not to attend?
It was:
If the doctor had attended, would she have intubated, and would intubation have avoided the injury?
That is “but for” reasoning.
For expert witnesses, Bolitho is a reminder that every link in the counterfactual chain matters. If the alleged breach is failure to assess, the expert must explain what assessment would probably have shown. If the alleged breach is failure to refer, the expert must explain what referral would probably have led to. If the alleged breach is failure to treat, the expert must explain what treatment would probably have achieved.
How Bailey Fits with “But For”
Bailey v Ministry of Defence is more difficult because it deals with cumulative causes. The claimant’s weakness resulted from both negligent and non-negligent factors. The weakness then led to aspiration and brain injury.
The teaching point is that some injuries are caused by a combination of factors that operate together. In those cases, it may not be possible to divide the injury neatly into separate parts. Where the negligent factor made a material contribution to the injury-producing process, causation may be established.
For expert witnesses, the important point is not to treat Bailey as a shortcut. It does not mean that any increase in risk proves causation. It means that where negligent and non-negligent factors combine to cause an indivisible injury, the expert should consider whether the negligent contribution was more than trivial.
The practical question becomes:
Did the breach materially contribute to the mechanism that produced the injury?
That is different from asking merely whether the breach increased the risk.
How Jones Helps Experts Avoid Guesswork
Jones v Great Western Railway is useful because it distinguishes inference from speculation.
Expert witnesses often have to say what probably would have happened in circumstances that were not properly documented. That is acceptable where the opinion is based on evidence. It is not acceptable where the expert is simply guessing.
A reasonable inference is grounded in facts. Speculation is not.
For example:
The patient would probably have been referred urgently because the documented symptoms met the urgent referral criteria.
That is an inference.
By contrast:
The patient would probably have been referred urgently because the clinician might have been worried.
That is speculation unless supported by the records.
In “but for” causation, the expert is often reconstructing an alternative clinical pathway. Jones is a reminder that this reconstruction must be evidence-based.
Final Teaching Summary
For expert witnesses, the phrase “but for” can be translated into plain English:
If the negligence had not happened, would the same injury probably still have occurred?
That is the question.
The expert should not be distracted by the unfamiliar wording. In practical terms, the task is to compare the actual clinical pathway with the probable non-negligent pathway.
The expert should ask:
- What actually happened?
- What should have happened?
- What would probably have happened if reasonable care had been provided?
- Would the same injury probably still have occurred?
- If not, what injury was probably caused or materially contributed to by the breach?
The safest expert reports do not merely state conclusions. They explain each step in the causal chain.
Bolitho teaches experts to analyse what would have happened if an omitted assessment or treatment had taken place.
Bailey teaches experts to consider material contribution where negligent and non-negligent factors combine to produce an indivisible injury.
Jones teaches experts to base counterfactual reasoning on evidence and reasonable inference, not speculation.
The expert witness does not need to sound like a lawyer. The expert does need to answer the legal question clearly.
The clearest formulation is often this:
Taking the breach away, I consider that the same outcome would probably still have occurred.
Or:
Taking the breach away, I consider that the same outcome would probably have been avoided.
That is “but for” causation in practical expert witness language.
To instruct an expert witness or to discuss your requirements, please contact Expert Court Reports via their website at www.expertcourtreports.co.uk, by telephone on 01865 587865 or by email at office@expertcourtreports.co.uk.
