Insulin Test Irregularities in Lucy Letby Case Raise Questions About Conviction

The case of Lucy Letby, the former neonatal nurse convicted of murdering seven infants and attempting to murder seven others at the Countess of Chester Hospital between June 2015 and June 2016, has been thrust back into the spotlight with the revelation of irregularities surrounding key insulin test evidence used in her trial. A new investigation has uncovered that a critical insulin test, pivotal to securing her conviction for the attempted murder of two infants, referred to as Baby F and Baby L, was taken two days later than initially presented. Furthermore, a second test result, hidden in the case file and not disclosed during the trial, suggests a far more complex and troubling timeline, casting doubt on the reliability of the prosecution’s evidence. This article explores the details of these findings, their implications for Letby’s conviction, and the broader questions they raise about the integrity of the case.

Background of the Lucy Letby Case

Lucy Letby, now 35, was found guilty in August 2023 of seven counts of murder and seven counts of attempted murder, receiving 15 whole-life sentences. The prosecution’s case relied heavily on circumstantial evidence, including Letby’s presence during a series of unexpected infant deaths and collapses, medical record inconsistencies, and handwritten notes interpreted as confessions. Among the most damning evidence were blood tests indicating insulin poisoning in two infants, Baby F and Baby L, which the prosecution described as a “smoking gun” proving deliberate harm. However, recent scrutiny of these tests, as reported by The Telegraph and The Guardian, reveals significant flaws that challenge the foundation of these convictions.

The Insulin Tests: A Critical Piece of Evidence

The prosecution alleged that Letby attempted to murder Baby F in August 2015 and Baby L in April 2016 by adding synthetic insulin to their intravenous feeding bags, causing dangerously low blood sugar levels (hypoglycemia). Blood tests showed high insulin levels and low C-peptide levels, a pattern the prosecution claimed could only result from external insulin administration, as C-peptide is a byproduct produced by the body in roughly equal amounts to natural insulin. The tests were conducted using an immunoassay method at Liverpool Clinical Laboratories, with results presented as definitive evidence of poisoning.

However, new findings indicate that the insulin test for Baby F was taken two days later than stated during the trial. Court testimony suggested the sample was collected shortly after the infant’s collapse on August 5, 2015, but medical records reviewed by experts, including those submitted to the Criminal Cases Review Commission (CCRC), show the sample was actually taken on August 7, 2015. This delay raises questions about the sample’s integrity, as insulin degrades over time in stored blood, potentially skewing results. Furthermore, a second test result for Baby F, discovered in the case file but not presented to the jury, showed inconsistent insulin and C-peptide levels that did not align with the prosecution’s poisoning narrative. This hidden result suggests either a testing error or an alternative explanation for the infant’s condition, such as natural metabolic fluctuations common in preterm infants.

Expert Challenges to the Insulin Evidence

Multiple experts have since criticized the reliability of the immunoassay tests used in the Letby case. Dr. Adel Ismail, a retired consultant in clinical biochemistry with 25 years of experience leading an NHS pathology lab, told The Telegraph that immunoassays are notoriously unreliable, with an error rate of at least one in 200, and possibly as high as one in 100. The tests use antibodies as reagents, which can interact with other antibodies in a patient’s blood, leading to false positives or negatives. Ismail emphasized that confirmatory testing, such as mass spectrometry, is recommended when insulin poisoning is suspected, but no such tests were conducted for either Baby F or Baby L.

Professor Geoff Chase, a leading expert on insulin in preterm infants from the University of Canterbury, and chemical engineer Helen Shannon further challenged the prosecution’s claims. Their 100-page report, submitted to the CCRC, concluded that the insulin levels reported were “typical” for preterm infants and that significantly higher doses—20 to 80 times more than alleged—would have been needed to cause harm. They also noted that no evidence of missing insulin stocks was presented at trial, undermining the feasibility of Letby’s alleged method.

Additionally, the prosecution’s expert, Dr. Anna Milan, testified that Baby F’s C-peptide level was below 169 picomoles per liter, suggesting it was “undetectable” and thus indicative of synthetic insulin. However, The Telegraph confirmed with Roche, the manufacturer of the immunoassay, that the test’s lower limit of detectability is three picomoles per liter, meaning the jury may have been misled about the significance of the C-peptide results. The confusion between “undetectable” and “unquantifiable” terms during the trial further muddied the waters, potentially influencing the jury’s interpretation of the evidence.

The Hidden Test Result and Strange Timeline

The undisclosed second test result for Baby F, buried in the case file, presents a “stranger timeline” that complicates the prosecution’s narrative. While the first test, taken two days late, showed high insulin and low C-peptide, the second test—conducted at an unspecified time—revealed a different pattern, with insulin and C-peptide levels that did not clearly support the poisoning hypothesis. Experts like Professor Joseph Wolfsdorf from Harvard Medical School have argued that the results, even if accurate, may not be consistent with insulin poisoning in preterm infants, whose physiology differs significantly from older children or adults.

The delayed testing and hidden result raise serious questions about the chain of custody and the prosecution’s transparency. The samples were not sent to a forensic laboratory, as would be standard in cases of suspected foul play, and were discarded after initial testing, preventing the defense from conducting independent analysis. Professor Alan Wayne Jones, a forensic toxicologist, told The New York Times that immunoassays are “not sufficient” for criminal prosecutions due to their high error rate and the need for stringent forensic protocols.

Implications for Letby’s Conviction

The insulin test irregularities are part of a broader wave of skepticism about Letby’s convictions, fueled by a growing number of medical and scientific experts. A panel of 14 experts, including neonatologist Professor Neena Modi, concluded in a report to the CCRC that there was “no medical evidence supporting malfeasance” in any of the 17 cases, attributing the infants’ deaths and collapses to natural causes or substandard care at the Countess of Chester Hospital. The hospital’s neonatal unit was understaffed, overworked, and plagued by issues like plumbing problems and bacterial outbreaks, which may have contributed to the spike in mortality in 2015–2016.

Letby’s new barrister, Mark McDonald, has described the case as “one of the major injustices of modern times,” arguing that the failure of her original legal team to call expert witnesses left the prosecution’s evidence unchallenged. The insulin test discrepancies, particularly the delayed test and hidden result, are now central to an ongoing CCRC review, which could refer the case back to the Court of Appeal if new evidence undermines the convictions’ safety.

Broader Systemic Issues

The revelations about the insulin tests highlight systemic flaws in the Letby case, from the handling of medical evidence to the hospital’s initial response to concerns. The Thirlwall Inquiry, ongoing as of August 2025, has exposed how senior consultants, including Dr. Stephen Brearey, raised alarms about Letby as early as June 2015, but hospital management resisted action, citing insufficient evidence. A leaked audio from a June 2016 meeting revealed a manager’s dismissive response—“There is no evidence”—to demands for Letby’s removal, underscoring a culture that prioritized reputation over patient safety.

The inquiry has also highlighted that two insulin poisoning cases were not escalated to senior consultants at the time, leaving them unaware of the full picture until after Letby’s arrest in 2018. This lack of transparency, combined with the reliance on flawed immunoassay tests, suggests a failure to rigorously investigate alternative explanations for the infants’ conditions, such as natural metabolic issues or hospital-acquired infections.

Public and Expert Reactions

The families of Letby’s victims have expressed profound distress over the questioning of her convictions, with Lady Justice Thirlwall, leading the public inquiry, condemning skeptics for causing “enormous additional distress.” However, experts like Dr. Ismail and Professor Chase argue they are compelled to speak out due to the scientific inaccuracies presented at trial. Public figures, including MP David Davis and former Supreme Court judge Lord Sumption, have called for an urgent re-examination of the case, citing concerns about the misuse of statistical and medical evidence.

The insulin test irregularities have drawn parallels to past cases, such as that of nurse Colin Norris, convicted in 2008 of insulin-related murders but whose case was referred back to the Court of Appeal due to similar concerns about unreliable immunoassays. Dr. Ismail has warned that history may be repeating itself, with laboratory tests being used as definitive proof despite their limitations.

Conclusion

The revelation that a key insulin test in the Lucy Letby case was taken two days late, coupled with a hidden second test result that suggests a conflicting timeline, has significantly undermined the prosecution’s evidence of insulin poisoning. These findings, supported by expert analyses questioning the reliability of immunoassay tests and the physiological plausibility of the allegations, highlight potential miscarriages of justice in Letby’s convictions for the attempted murders of Baby F and Baby L. As the CCRC reviews the case and the Thirlwall Inquiry continues, these irregularities underscore the need for greater scrutiny of medical evidence in criminal trials and a reevaluation of how hospitals and legal systems respond to complex medical cases. For the families, the ongoing debate is a painful reminder of their loss, while for the justice system, it is a call to ensure that convictions are based on robust, transparent evidence.

Sources: The Telegraph, The Guardian, The New York Times, BBC News, The Independent