LUCY LETBY CASE: NEW SCIENTIFIC CHALLENGE PUTS INSULIN EVIDENCE UNDER THE MICROSCOPE

A major new scientific challenge has put one of the most important pieces of evidence in the Lucy Letby case back under intense scrutiny.

Letby, the former neonatal nurse convicted of murdering seven babies and attempting to murder seven others at the Countess of Chester Hospital, is serving whole-life prison terms while continuing to maintain her innocence. Her convictions are currently being reviewed by the Criminal Cases Review Commission (CCRC).

Now, fresh expert analysis submitted to the CCRC is questioning aspects of the medical evidence used to argue that two babies were deliberately poisoned with insulin.

THE INSULIN EVIDENCE AT THE CENTER OF THE FIGHT

At Letby’s trial, prosecutors argued that unusually high insulin levels combined with unusually low C-peptide levels showed that insulin had been administered externally.

That evidence was central to the allegations involving two premature babies.

But new research and expert reports have challenged whether the laboratory findings can be interpreted as conclusively as the jury was told.

A 2026 report by neonatal expert Neil Aiton and statistician Hilde Wilkinson-Herbots was submitted to the CCRC, raising allegations of factual errors and omissions in the evidence given by prosecution expert Professor Peter Hindmarsh.

Separately, newly published research has raised questions about how insulin measurements in premature infants should be interpreted, particularly in the context of serious illness.

COULD THERE HAVE BEEN OTHER EXPLANATIONS?

This is where the scientific dispute becomes critical.

Letby’s defense has argued that the babies’ abnormal biochemical results may have had natural or medical explanations, rather than necessarily proving deliberate insulin administration.

Some experts have pointed to conditions affecting premature and seriously ill infants as possible alternative explanations that should be considered when interpreting insulin and C-peptide results.

However, that does not mean scientists have established that the babies were not poisoned.

It means the interpretation of the medical evidence is being challenged — and whether those challenges are sufficient to undermine the convictions is ultimately a matter for the courts.

 ANOTHER BOMBSHELL: A PROSECUTION EXPERT WAS UNDER INVESTIGATION

The scientific controversy was followed by another development that attracted major attention.

The Guardian reported in March 2026 that Professor Peter Hindmarsh, a key prosecution expert who testified about the insulin cases, was subject to a professional investigation by the hospital employing him while he was involved in the Letby proceedings.

The jury was not told about that investigation during the trial. Cheshire police and the Crown Prosecution Service said they had not been informed of it before Hindmarsh gave evidence.

That revelation has prompted further questions about the expert evidence presented to the jury.

It does not, by itself, establish that Hindmarsh’s testimony was incorrect or that Letby’s convictions are unsafe.

But it is now part of the wider scrutiny surrounding the case.

 WHAT DOES THE CCRC HAVE TO DO WITH IT?

The CCRC is not a court and cannot declare Letby innocent.

Its role is to investigate whether new evidence or new arguments create a real possibility that a conviction would not be upheld if the case returned to an appellate court.

The commission has explicitly stressed that its review is impartial and evidence-based, and that determining guilt or innocence remains the responsibility of the courts.

If the CCRC concludes that the legal threshold is met, it can refer the convictions to the Court of Appeal.

Until then, Letby’s convictions remain in force.

 THE BIGGER QUESTION

The Letby case has always depended heavily on medical and circumstantial evidence.

There was no eyewitness who testified to seeing Letby inject insulin into a baby’s bloodstream.

Instead, prosecutors presented a combination of medical findings, laboratory results, staffing information, clinical events and other circumstantial evidence to build their case.

Now, several years after the original convictions, defense lawyers are asking whether some of that evidence was interpreted correctly — and whether the jury had been given the full range of reasonable medical explanations.

That distinction matters.

A scientific disagreement does not automatically overturn a conviction.

But if new evidence demonstrates that a crucial piece of evidence was scientifically unreliable, incomplete or capable of a substantially different interpretation, the legal consequences could be significant.

 THE CASE IS NOT OVER

For now, there has been no ruling that Lucy Letby is innocent, and no court has overturned her convictions.

But the case has entered a new phase in which the science behind some of the prosecution’s most important claims is being examined again.

And that leaves one question hanging over the entire investigation:

If the medical evidence can legitimately be interpreted in more than one way, what does that mean for the convictions that depended upon it?

The CCRC’s eventual decision could determine whether those questions remain matters of public debate — or return to the Court of Appeal for a formal legal examination.