July 2026: Government declines to widen the Thirlwall terms of reference (16 July) · new 100-page insulin report to the CCRC challenging the trial evidence (9 July) · Thirlwall report still expected no earlier than September · inquests relisted to 2027 · Shoo Lee Panel: no medical evidence of deliberate harm.
Medical explainer
C-peptide is produced when the pancreas makes its own insulin; injected (exogenous) insulin contains no C-peptide. A low insulin-to-C-peptide ratio is therefore presumptive of exogenous insulin — but only when the immunoassay used is forensically validated and confirmatory testing (mass spectrometry) is performed.
Two attempted-murder counts in the Letby case rested on insulin-to-C-peptide ratios from the Royal Liverpool laboratory. Named clinical biochemists have argued the immunoassay was not validated for forensic interpretation, that mass spectrometry was not performed, and that interfering antibodies and the immunoassay 'hook effect' were not excluded.
The Crown treated the laboratory ratio as forensic proof of exogenous insulin administration.
Prof. Adel Ismail, Prof. Geoff Chase and others have argued for the necessity of confirmatory mass spectrometry in forensic insulin determination. The Joint Insulin Report (May 2025) consolidated this view.
No TPN bags were retained or tested, so there is no physical evidence either way on the alleged route of administration. Treatment for hypoglycaemia itself suppresses C-peptide, and the samples were taken after repeated dextrose infusions, which means the baseline the ratio was read against is itself uncertain.
C-peptide is produced when the pancreas makes its own insulin. Injected insulin contains none. A low insulin-to-C-peptide ratio is therefore presumptive of insulin from outside the body — but only if the assay is forensically validated and confirmatory testing is done.
An immunoassay measures insulin indirectly, by antibody binding. It is fast and sensitive but susceptible to cross-reactivity and interference. Forensic interpretation normally requires confirmatory mass spectrometry, which identifies insulin by mass-to-charge ratio instead.
No. The screening immunoassay result was not confirmed by the mass spectrometry the assay manufacturer's own guidance recommends for forensic use.
That their insulin/C-peptide ratios were within the range normal for preterm infants and do not prove exogenous insulin. It attributed Child F's prolonged hypoglycaemia to sepsis, prematurity, intrauterine growth restriction, a tissued long line and poor management, and Child L's to preterm birth and severe intrauterine growth restriction, inadequately managed.
Independent commentary identifies the Royal Surrey laboratory at Guildford as the UK forensic standard. The Countess of Chester samples went to the Royal Liverpool clinical biochemistry laboratory, which does not operate under the Forensic Science Regulator's code of practice.
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