When Lucy Letby was questioned during her trial about how two babies came to have dangerously low blood sugar, even she accepted that they must have been poisoned with insulin.

Blood test results appeared to show the insulin levels in the babies were not natural, suggesting the hormone could only have been administered externally.

Now experts are casting doubt not only on whether Letby was guilty, but whether extra insulin was ever given to the babies at all.

Prof Geoff Chase and Helen Shannon, a chemical engineer, have written an extensive 173-page report for Letby’s defence team, which has been submitted to the Criminal Cases Review Commission (CCRC), the body that examines potential miscarriages of justice.

“I am happy to show up in court, or to the CCRC, or the Crown Prosecution Service, and go through it with them line-by-line,” said Prof Chase, a specialist in insulin delivery to critically ill adults and neonates, and author of over 650 journal articles, from the University of Canterbury in Christchurch, New Zealand.

Here, they set out their arguments to The Telegraph.

The missing insulin

Letby was convicted of injecting insulin into the feed bags of two babies known as Child F and Child L at the Countess of Chester Hospital in 2015 and 2016.

Both had suffered alarming crashes in blood sugar, and medical staff were sufficiently concerned to send blood samples to a laboratory in Liverpool for analysis.

The lab flagged an anomaly in both cases. When the body creates insulin naturally, it also produces the same amount of a second hormone, called c-peptide, which breaks down at an equal rate.

Because insulin leaves the body much faster than c-peptide, there should have been far less of it – but in the case of Child F, the lab found the reverse. Tests showed high insulin but low c-peptide.

Child L also had relatively high insulin levels, and both cases suggested extra insulin had been administered accidentally or, as the prosecution would claim, maliciously.

Both babies recovered before the tests returned, so the hospital filed away the reports, which were not found until detectives began hunting for evidence and noticed that the incidents coincided with Letby’s shift patterns.

During Letby’s trial, Prof Peter Hindmarsh, an endocrinologist from University College London (UCL), calculated that just a drop (0.6ml) of insulin needed to be added to feed bags to cause the crashes.

But Prof Chase and Ms Shannon disagree. According to them, it would require at least 10 times as much (6ml) to produce the same effect – nearly two thirds of a vial of insulin. This is because insulin is ‘sticky’, holding on to surfaces rather than being pumped into the body.

They calculate it could even require up to six to seven vials, which would have been noticed by hospital staff. Yet no insulin was ever found to be missing.

“Insulin sticks, or technically adsorbs, to many surfaces and particularly plastics used in the bags delivering nutrition as well as the lines and cannulas,” said Ms Shannon.

“The trial stated no insulin was missing and it would require only a very small drop. If injected intravenously yes, this is true, but with adsorption, or sticking, it would require between 1.2 teaspoons to six to seven vials.

“Simply, there was no possibility to commit insulin poisoning as hypothesised in court.”

Letby’s trial heard that just three vials of insulin were issued to the neo-natal unit in 2014, six vials in 2015 and two vials in 2016 and were kept in a locked fridge in an equipment room.

Prof Chase added: “If no insulin is missing, there is no way to commit that crime. Insulin sticking to lines, bags, tubes is a known thing and it can significantly impact, for example, insulin pump use among small children. The lack of missing insulin is the true smoking gun in this case.”

The problem with antibodies

Even if there was no missing insulin, it still does not explain why the lab readings for Child F and Child L looked so worrying.

At Letby’s trial, Dr Anna Milan, a biochemist at the Liverpool lab, told the court that “the only way you can get a pattern like that is if there’s insulin [that has] been given to a patient”.

Prof Chase and Ms Shannon dispute this. They argue that babies are often born with antibodies that bind with insulin, effectively storing the hormone in the body for much longer than normal, and raising levels ‘beyond expectation’.

Studies have shown that between three and 97 per cent of preterm infants are born with such antibodies, and it is even more likely if they have been exposed to infections and some antibiotics.

In the case of the Letby babies, both had been treated with antibiotics for suspected sepsis, and the neo-natal unit had been struggling to get rid of a bacterium – Pseudomonas aeruginosa – which had colonised taps.

The unit was also having a problem with sewage leaking from pipes in the ceiling, and had been forced to put nappy pads in the roof cavity to soak up the waste. The risk of infection in the unit was high.

“When a neonate or its mother is exposed to infective agents, or certain antibiotics, insulin levels can be sky high,” Prof Chase added. “When an insulin antibody binds to insulin, what happens is it stays in the blood, and it stores it, and it’ll stay there for days, to weeks, to months. In one published case study it took a year for bound insulin to disappear.”

The antibody problem is well known, and the lab should have sent the sample away for further testing but that never happened.

“If you go to a variety of online NHS guidelines around insulin and c-peptide testing they will tell you that if you get a very high insulin rating, you should test again, and they’ll also tell you to test for antibodies,” said Prof Chase. “But that was never done because [the result] wasn’t important to people at the time.”

An NHS guidance note from the Liverpool lab warned that the Roche assay used could not determine whether insulin had been artificially administered and advised sending it to a forensic lab

But samples were never checked by forensic experts and the defence was not able to re-test the blood. It had already been thrown away.

Preterm anomalies

The prosecution’s case was that the high levels of insulin and low c-peptide indicated foul play.

In the trial, it was stated that a healthy insulin to c-peptide ratio should be between 1:10 and 1:20. In other words, there should be 10 to 20 times more c-peptide than insulin.

In the case of Child F, that ratio was flipped, with insulin levels of 4,657 and a very low c-peptide level of less than 169.

But Prof Chase and Ms Shannon said the trial experts were relying on data from older children or adults, and that newborn and preterm babies often have strikingly different hormone ratios.

In fact, the c-peptide levels for all the infants were well within reported ranges for neonates in published studies.

One paper, which looked at 76 sets of assays from preterm infants, showed more than over 40 per cent had hormone levels that were equal or even reversed, with insulin higher than c-peptide.

And their new modelling showed that when antibody binding was added into the mix, insulin levels could rise up to 10,000.

Prof Chase said: “Normally insulin and c-peptide come out in equal amounts. C-peptide goes up into a bucket with one hole, insulin goes into a bucket with three holes.

“But if insulin also binds with antibodies and goes into a bucket with no holes, the relative levels can change and measured insulin levels build up over time. And once you have antibodies, you can get almost any number you want in relatively short order.”

Ms Shannon added: “If you’ve got these infective agents in the mix, and when you’ve got insulin binding reactions going on, virtually any neonate would react in the way these neonates did.”

“Critically, the Roche assay used measures this bound insulin, which is why retesting for antibodies or more specific tests are required to determine insulin poisoning.”

The pair also argue that it is not uncommon for preterm babies to develop low blood sugar, particularly if they are critically ill. Research shows that around one in three such infants suffer very low blood glucose levels and up to half are not responsive to glucose.

‘No way to prove a crime occurred’

Even during the trial opening, Nicholas Johnson KC admitted the insulin anomalies were originally “attributed to a naturally occurring phenomenon”.

Ms Shannon added: “There is no way to prove a crime occurred and there is ample evidence to suggest non-malicious interpretation of the assay results.”

The assays themselves are also not foolproof, with up to four per cent giving misleading or unreliable results.

The lab that tested the samples underwent an assessment a few weeks after Child L’s test, which found it exaggerated the level of insulin in a quality control sample by almost 800 per cent.

Prof Chase added: “None of this is hard to find. But seemingly it didn’t cross anyone’s minds to check with 15 life sentences on the line.”

The police and Crown Prosecution Service said there was a wealth of evidence, and testimony from medical experts, to convict Letby, and are considering bringing new charges against the nurse. The Liverpool lab is also standing by its findings.

Meanwhile, the CCRC is deciding whether to send Letby’s case back to the Court of Appeal, with a decision expected before Christmas.

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