Conspiracy theories have swirled around the former nurse convicted of murdering and attempting to kill babies at Countess of Chester Hospital, but it’s not just armchair detectives who are uneasy – experts from neonatology to statistics have also voiced concerns about the convictions, say Tom Ball and Tom Whipple in the Times. They lay out some of the key areas that have been raised as points of concern:
Insulin testing
Letby was convicted of killing and harming babies using two main methods: injecting air into their bloodstreams and poisoning them with insulin.
Although the case for the former relied largely on a combination of indirect and circumstantial evidence, the prosecution had empirical scientific evidence for the latter in the form of test results showing that two babies had suffered from sudden unexplained drops in their blood sugar, a condition known as hypoglycaemia.
Letby was found guilty of attempting to murder these two babies by injecting synthetic insulin into their feed bags. Both babies survived.
These test results were the closest thing prosecutors had to a smoking gun, and were a keystone for their case as a whole. Their argument ran that if the jury could agree that Letby had deliberately poisoned two babies, they could also reasonably conclude that she had harmed others using different methods, even if the evidence for those were less concrete. …
In court Dr. Anna Milan, a biochemist at the Royal Liverpool Hospital, where the tests were carried out, said the results had shown that insulin had been given to the patients rather than being produced by the pancreas.
She explained that this was evidenced by the fact that tests showed the two babies had very high levels of insulin in their blood, but only a negligible amount of C-peptide. …
C-peptide is produced with insulin in the body. It therefore follows that if there is no C-peptide present, the insulin has not been produced naturally and must have been introduced from outside. However, although this test – known as an immunoassay – is a useful guide for diagnosing hypoglycaemia, it does not test for the presence of insulin itself.
As the Royal Liverpool recommends on its own website, if external insulin administration is suspected, a separate, more specific, analytical method is recommended. This did not happen in the Letby case, however, because both babies recovered soon afterwards. …
Air embolism
The other principal method with which Letby was convicted of harming seven babies was the injection of air into the bloodstream, causing what is known as an air embolism. In these cases babies exhibited an unusual skin discolouration shortly after they collapsed, which doctors on the unit said they had never seen before.
Dr. Dewi Evans, the lead expert witness called by the prosecution, drew on these descriptions in concluding that the babies had suffered from air embolisms. His opinion was supported by Dr. Sandie Bohin, the second expert medical witness, who was also given access to medical documents.
Evans, a paediatric consultant with more than 30 years experience as an expert witness, cited a paper written in 1989 by Dr. Shoo Lee, a Canadian neonatologist, which detailed 53 cases of air embolism in newborn babies, some of whom had also shown signs of skin discolouration.
Lee, who recently retired, was not called to give evidence by either the prosecution or defence, though he did appear at Letby’s appeal hearing in April. Rather than confirming Evans’s and Bohin’s diagnosis, he told the Court of Appeal that none of the descriptions of the babies’ skin discolourations given by witnesses matched the sort that he had recorded in his paper. …
The judges, however, did not admit his testimony, saying that the defence might have called him in the trial but chose not to. …
Dr. Michael Hall, who was a medical expert witness for the defence, said that in his view the charge of air embolism had not been proved beyond all reasonable doubt.
He said that another research paper on the effects of air embolism in neonates had described a quite different discolouration, in which the baby’s skin had gone blue and stayed blue for some hours.
He also said that Lee’s paper itself was not a suitable yardstick by which to judge air embolism in the Countess of Chester babies as the research had looked at a different scenario, in which oxygen had been delivered by force through ventilation. …
‘Easy to over-interpret the data’
At the very start of the trial, the jury were shown a chart listing 25 suspicious deaths and collapses between June 2015 and June 2016 on one axis, and the names of the 38 nurses who had worked on the unit on the other.
Every other nurse had a handful of crosses showing that they were on duty during incidents, but Letby had an unbroken row of crosses beside her name, putting her at the scene for every death and collapse.
However, the table did not include six other deaths during that period, for which Letby was not charged. If it had, the results would have appeared more mixed and the evidence against Letby less damning.
Professor Peter Green is a statistician at Bristol University. He was part of a group that wrote a report published by the Royal Statistical Society (RSS) about the use of statistics in cases of suspected medical misconduct. Released shortly before the Letby trial, it was titled ‘Healthcare serial killer or coincidence?’
Speaking in a personal capacity, he said it was clear that the standards the group recommended had not been applied in presenting Letby’s shift roster to the court. “Whatever the truth here, it is clear that the investigation and prosecution did not follow the good practice laid out in our report – and also that the defence may have missed some opportunities,” he said. “It’s easy to over-interpret this kind of data. People are very good at seeing patterns.”
Although the statistical evidence was only one part of the trial, he said, his worry was how it might have influenced the interpretation of medical evidence.
“The principal problem is the failure to account for other explanations,” he said. “There’s also a sense that by summarising it in such a succinct way you are capturing the whole story, and that’s very, very compelling. People like simple explanations. And of course, people also like to blame human culprits, not problems with systems.” …
‘Context is everything’
Uncertainties were an acknowledged part of the case against Letby. During the original trial, the judge told the jury that it was not necessary for the prosecution to prove the precise manner in which she had acted, only that she had acted with murderous intent.
In doing so, the prosecution did not rely solely on medical and statistical evidence. It presented circumstantial evidence, such as the Letby’s handwritten notes saying “I am evil, I did this”.
As Nick Johnson KC, for the prosecution, told the court: “In this extraordinary case, context is everything.” In other words, there was no one piece of evidence that proved Letby to be a killer beyond all reasonable doubt. Rather, guilt was evinced by a combination of interlocking, co-dependent facts.
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