Scientific evidence of how nurse Lucy Letby was said to have harmed a number of babies was “so poor” it cannot be “safely used” to support the allegations, a court has heard.

It is claimed Letby, 33, targeted a number of infants at the Countess of Chester Hospital’s neonatal unit by injecting air into their bloodstream.

The prosecution say this caused an air embolism which blocked their blood supply and led to sudden and unexpected collapse, with some proving fatal.

On Tuesday, Ben Myers KC, continued his defence closing speech at Manchester Crown Court and asked the jury of eight women and four men to consider how the theory of air embolus worked in this case.

He said: “This is meant to be reliable, scientific medical theory, underpinning the most serious allegations.

As a basis for conviction for someone of murder and attempted murder it is tenuous in the extreme

“At the heart of it are prosecution experts Dr Dewi Evans and Dr Sandie Bohin. They are the ones we say are pushing it and the prosecution rely on it, of course.

“Neither of them has clinical experience in identifying or treating air embolus.”

He said both had principally relied on a research paper written more than 30 years ago about the effect of air embolism on infants.

That study, said Mr Myers, showed 11% of 53 children had displayed signs of skin discolouration.

In several cases there were “blanching and migrating areas of cutaneous pallor”, the court heard, and in one case there was “bright pink vessels against a generally cyanosed cutaneous background”.

Mr Myers told jurors: “As a basis for conviction for someone of murder and attempted murder it is tenuous in the extreme.”

He added: “That meagre piece of research has carried into guesswork in this case.”

Mr Myers said both experts had identified five clinical features to support the identification of an air embolus – the presence of an intravenous (IV) cannula, a sudden and unexpected collapse, unusual skin discolouration, the presence of air in the great vessels of the heart and that resuscitation was unsuccessful.

He added: “Apart from needing to have an IV entry point, we say not one of those criteria has been applied consistently … during this trial.

“They (the experts) have chopped and changed them as much as required to fit the available evidence.

“Extraordinary contortions made to fit that theory.”

He said there were “many causes” of discolouration in a baby and in this case there was no precise record taken such as a photograph.

The descriptions varied between witnesses, he said, and sometimes came months and years afterwards, following discussions with other witnesses.

Mr Myers said: “The dangers of recollection being contaminated and influenced are obvious.”

He said the number of babies in this case who recovered from an alleged injection of air “does not make sense”, and that any neonate had the “medical potential” to deteriorate suddenly and unexpectedly.

Air in the great vessels of the heart does not in itself diagnose a gas embolus, Mr Myers said.

He told jurors: “Scientific evidence needs to be sufficiently reliable if you are going to rely on it.

“What guidance you have had from the experts has been applied inconsistently throughout the case.

“The evidence is so poor it cannot be safely used to support these allegations.”

Letby, from Hereford, denies the murders of seven babies and the attempted murders of 10 others between June 2015 and June 2016.

The trial continues on Wednesday.