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Literature and logic – the need for both when adducing expert evidence in clinical negligence claims

09 June 2022

In Pickering v Cambridge University Hospital NHS Foundation Trust [2022] the High Court, and specifically presiding Judge, Mr Justice Richie reaffirmed the importance of persuasive, logical and clear expert evidence in clinical negligence cases. 

This decision stands as a reminder to parties involved in clinical negligence litigation that the credibility of expert witnesses can stand or fall on their ability to provide rational and coherent evidence, making necessary concessions where appropriate.  

The Facts

Ms Pickering, was a patient with longstanding Atrial Fibrillation (AF- an irregular heart rhythm associated with blood clots in the heart) who sadly went on to suffer a significant stroke on 27 September 2015. Ms Pickering had been diagnosed with AF for many years prior to 2015 but had opted not to have the recommended therapeutic anti-coagulation treatment (Heparin) to reduce her risk of stroke leading up to the events, instead favouring Aspirin. 

On 24 September 2015 Ms Pickering attended Addenbrooke's hospital with symptoms relating to her right lower leg being extremely cold, white and with paraesthesia. By the time of her assessment in A&E her symptoms had resolved and she was discharged with safety netting advice, but without treatment with Heparin. At 20:42 on 27 September 2015 Ms Pickering went on to suffer a serious stroke with a clot in the left Middle Cerebral Artery and affecting the left carotid artery. Ischaemic brain injury continued over the following 2 days.

The Issues

Following breach of duty admissions by the Defendant, it was thereafter not in dispute that the Defendant should have prescribed the Claimant Heparin before 01:44 on 25 September 2015.

The trial continued on causation. It was not disputed that the Claimant's stroke arose with a clot forming in the left atrium of the heart and a small embolus having "fired off" into the brain; the very same mechanism which had led to the clot in the Claimant's right leg on 24 September. The key causation question for determination was whether, but for the Defendant's negligence, the Claimant would not have suffered a stroke had she been treated with Heparin on 25 September 2015.  The case was defended by the Trust on the basis that this 48 hour window of opportunity was too short for Heparin to make any material difference and avoid the stroke. 

Mr Justice Ritchie heard evidence from 4 causation experts at trial, with the main focus being on the Haematologists, Professor Mehta for the Claimant and Dr Patel for the Defendant. Lengthy evidence was heard with the Professor Mehta, drawing indirectly upon medical literature dealing with the efficacy of Heparin in successfully avoiding acute cardio-embolic stroke. Professor Mehta accepted that there was a lack of direct evidence to support his opinion as to the success of Heparin in an acute emergency setting in patients with AF. However, Mr Justice Ritchie was not deterred in finding for the Claimant in this case notwithstanding the need to extrapolate from the existing literature and apply logical interpretation to the same. 

Professor Mehta opined that the immediate use of Heparin worldwide in treating acute AF is established and significantly reduces the risk of embolic stroke. The Defendant's haematology expert, Dr Patel, whilst accepting that Heparin was over 90% effective in preventing Deep Vein Thrombosis and Pulmonary Embolism within 2-3 days of treatment, maintained his position that the efficacy of Heparin in those scenarios could not be extrapolated to the Claimant's left atrial thrombosis. He asserted that venous clots were more amenable to Heparin whilst arterial clots could not be comparable and the lack of definitive evidence of any beneficial effect of Heparin in arterial thrombo-embolism must mean the Claimant's case failed. 

The Decision

Mr Justice Ritchie struggled with the rationale of Dr Patel's evidence in the face of his acceptance that Heparin prevents emboli in DVT/PE within 2-3 days due to rapidly reducing the size of the clot and "walling off", alongside his acknowledgement that Heparin prevents recurrent venous thrombosis and his assertion that the body then prevents clots causing further emboli. The fact that Dr Patel was not prepared to entertain the rationale that as any clot becomes smaller, more "organised or stabilised or adherent"  it becomes less likely to fire off emboli caused Mr Justice Ritchie to conclude that Dr Patel's logic was "questionable" with "fixed thinking" which undermined his credibility. The evidence of Professor Mehta was therefore preferred.

In finding for the Claimant, Mr Justice Ritchie stressed that the lack of definitive evidence in the medical literature relating to arterial thrombo-embolisms in an emergency setting was not a bar to his findings; here, as is often the case in clinical negligence claims, there could be no ethical way of obtaining data for comparison purposes. 


The decision provides an important reminder for those involved in clinical negligence cases that;

  1. Experts need to apply their experience and logic to the clinical scenario in issue, and
  2. Literature can be a flexible friend, and rigid interpretation can trip up an expert who fails to acknowledge the evidential gaps and need for extrapolation.  

Further Reading