Insulin poisoning — a screening assay used as forensic proof
Prosecution claim
Blood samples from two infants — Children F and L — returned results suggesting raised insulin with low C-peptide. Normally insulin and C-peptide are released together by the pancreas. A high-insulin-low-C-peptide pattern, the prosecution argued, is only explicable by insulin administered from outside the body. The jury was told this was proof of deliberate poisoning.
Counter-evidence
The Roche Cobas immunoassay used is a screening test. Its own manufacturer's guidance requires confirmation by mass spectrometry before a result can be treated as diagnostic of exogenous insulin. That confirmation was never done. Independent endocrinologists (including Adel Ismail and contributors to science4justice.nl) have shown the assay is prone to false positives in neonates because of interfering antibodies and cross-reactivity. The Panel concluded the insulin evidence cannot bear the weight placed on it in a criminal trial.
A screening immunoassay was never designed for forensic use. In every other British criminal case involving insulin, confirmatory mass spectrometry is performed. That did not happen here.
What the jury heard
Biochemists from the Royal Liverpool laboratory described the Roche immunoassay results as showing an insulin level inconsistent with endogenous pancreatic release. The Crown's witnesses told the jury the pattern left no explanation other than exogenous insulin in the TPN bag.
What the Panel says
The Panel notes the screening result was never confirmed by the confirmatory mass spectrometry that the assay manufacturer itself recommends for forensic use. Without that second test the result cannot discriminate reliably between exogenous insulin, interference from auto-antibodies, and certain cross-reacting compounds that are elevated in sick neonates. The Panel considers the evidence falls below the standard required for a murder or attempted-murder conviction.
What independent experts add
- Dr Adel Ismail, a clinical biochemist, has published detailed commentary on interference and false-positive rates in the Cobas assay in neonates.
- Prof. Geoff Chase (University of Canterbury NZ) has modelled the physiological plausibility of the reported values and concludes several alternative explanations are consistent with the observed data.
- science4justice.nl catalogues every known false-positive case in the assay's published literature.
- The Royal Liverpool laboratory's own records note that these samples were flagged as 'for clinical purposes only — not forensic'.