Context
Prof. Peter Hindmarsh gave paediatric-endocrinology expert evidence for the prosecution at the 2022–2023 Letby trial on the insulin counts for Babies F and L. Prof. Hindmarsh is Professor of Paediatric Endocrinology at University College London and Consultant at Great Ormond Street Hospital — a senior UK paediatric endocrinologist. He interpreted the Roche Cobas screening-immunoassay results produced by the Royal Liverpool clinical biochemistry laboratory as diagnostic of exogenous insulin administration.
Full contemporaneous testimony is archived in the Chester Standard live-blog coverage. This page is a structured reference summary; the May 2025 Joint Expert Witness Insulin Report on Babies F and L is the authoritative post-conviction response.
What Prof. Hindmarsh testified to
- His interpretation of the Roche Cobas insulin-immunoassay results for Baby F and Baby L as diagnostic of exogenous insulin administration on the basis of high-insulin-low-C-peptide patterns.
- The physiological framework for endogenous-vs-exogenous insulin discrimination in the absence of confirmatory testing.
- The numerical insulin and C-peptide values reported by the laboratory, treated as quantitatively reliable in the range reported.
The post-conviction clinical-biochemistry position
The Joint Expert Witness Insulin Report on Babies F and L (May 2025) consolidates the peer-reviewed literature and applies it to the trial record. Key post-conviction evidential challenges the Report addresses:
- The Roche Cobas screening immunoassay’s manufacturer documentation requires confirmatory mass-spectrometry testing for forensic use. That confirmation was not performed.
- The Baby F reading of 4,657 pmol/L is on the order of adult attempted-suicide values and is physiologically implausible on the prosecution’s own theory of a small-volume spike in a slow-running TPN bag.
- The Roche Cobas hook effect at high C-peptide concentrations produces non-linear assay responses that can be misread as exogenous-insulin-dominant.
- Baby F’s mother likely had gestational diabetes; insulin auto-antibody transfer from mother to infant can cross-react with the immunoassay.
- Baby L’s mother had Type 1 diabetes; the auto-antibody transfer window is longer in T1 maternal diabetes.
- The UK forensic-standard laboratory for insulin assay is the Royal Surrey County Hospital at Guildford, not Royal Liverpool clinical biochemistry. Royal Liverpool does not operate under the Forensic Science Regulator’s code of practice.
- Sample-handling protocols at Royal Liverpool in 2015–2016 (gel-tube collection, delayed centrifugation) did not meet the forensic sample-handling standard.
- No TPN bags were retained or tested. The prosecution theory required Ms Letby to have ‘spiked’ TPN stock accessible to all nursing staff on the unit.
Dr Adel Ismail (retired consultant clinical biochemist), Prof. Geoff Chase (biomedical engineer, University of Canterbury NZ), and the science4justice.nl insulin-question series (October 2023 onwards) provide the peer-reviewed architecture the Joint Report consolidates.
Read alongside
Prof. Peter Hindmarsh — biography, Evidence: core insulin issue, Evidence: the insulin hook effect, Evidence: Guildford forensic-laboratory standard, Evidence: TPN bag chain of custody, Evidence: insulin assay literature, Transcript: Panel Joint Insulin Report summary, Dr Adel Ismail — biography, Prof. Geoff Chase — biography, Analysis: Baby F insulin deep dive.