What are the odds? Probability versus Chance

Clyde & Co successfully defended a claim for substantial damages which arose from a decision not to offer the Claimant thrombolysis on 16.11.2016 when he presented to Hospital with symptoms which he suggested was a stroke.

Despite the Court finding the clinical documentation was  below a reasonable standard,  it found that he was not presenting with significant symptoms and his  stroke score was correctly assessed.  In any event, the claim failed on causation as even if he had been given thrombolysis, it would not have altered the outcome. He only lost the chance of a better outcome, which is not recoverable in clinical negligence claims.  

Issues

Signs versus Symptoms

The Court drew a distinction between signs and symptoms. Signs are objective findings made by the physician on examination, whereas symptoms are subjective experiences reported by the patient. The account given by the Claimant and his wife of the symptoms he suffered on had unconsciously become more detailed in their memories as time passed. The objective signs of a stroke, as opposed to the symptoms reported by the Claimant, were still too subtle. The Court also noted the Hospital’s protocol that a previous stroke within 3 months (which the Claimant had suffered in September 2016) was an absolute contra-indication for further thrombolysis.

Odds Ratio

It was common ground between the parties that the Emberson paper was trying to establish the benefit of giving thrombolysis, depending on the number of hours that had elapsed after the onset of symptoms. The Claimant’s stroke expert considered that the window is 3.5 hours from the onset of the stroke and if thrombolysis is given within this time frame, there would probably be a good outcome.  In cross examination, his reliance on the Emberson paper fell away as he conceded the difference in principle between an odds ratio (a way to compare the relative odds of an event happening in two groups) and the probability of an event occurring. 

To illustrate the point, Counsel for the Defendant put the following example to him in cross examination: if 33 out of 100 patients in a group had a better outcome with thrombolysis, than the 23 in a group who had not been treated with it, then it could not be said that the Claimant, on the balance of probabilities, would have had a better outcome – the most that he could say is that he lost the chance of a better outcome.  Put another way, as there was only a 10% increase from 23% to 33%, the Claimant’s expert had to accept  the outcome would have been the same. Ever since the House of Lords decision in Greg v Scott [2005] UKHL 2, damages for loss of a chance are non-recoverable.

That left the Claimant’s expert relying on the DRAGON score, which he suggested was a useful tool to determine in advance the likely outcome of treating a patient with thrombolysis. The problem though, as identified by the Defendant’s expert, was that it only deals with patients who have been treated with thrombolysis and therefore it could not enable the Court to conclude the Claimant would have had a better outcome. 

Discussion

There are often difficulties faced by both experts and practitioners in understanding not just what a particular medical study or paper is trying to achieve, but how it arrives at its conclusions. Parties relying on literature need to consider whether the statistical foundation for any particular study actually does support their case  on causation. A simple definition of an odds ratio is a statistic that quantifies the strength of the association between two events. The example given by the Defendant’s Counsel in cross examination illustrated there was only a 10% loss of a chance between outcomes for patients in the group that received thrombolysis and patients in the group that did not. Once the Claimant’s expert conceded this, his reliance on the DRAGON scoring tool emphasised the point even further – it had no comparative control group to evidence the efficacy of thrombolysis in the context of causation.