Why this count matters more than the others
Most counts on the indictment rest on expert interpretation of clinical signs. The Baby F insulin count is different: it has a numerical laboratory measurement. If the laboratory measurement is a sound forensic test, the count is evidentially stronger than any other. If it is not, the count collapses entirely.
The Crown told the jury “you cannot argue with a lab result”. The defence and the post-conviction independent experts say you can, and must, argue with this particular lab result.
Step 1: What the Roche Cobas immunoassay is
The Roche Cobas insulin immunoassay is a screening test. It uses antibody binding to estimate insulin concentration in a blood sample. It is designed to flag samples that might warrant further investigation, not to provide forensic-standard evidence of the type of insulin present in the sample.
For forensic use — distinguishing endogenous from exogenous insulin — the manufacturer’s own guidance requires confirmation by mass spectrometry. Mass spectrometry can separate the specific molecular forms of insulin and identify administered (synthetic) insulin versus pancreatic (endogenous) insulin. Immunoassay cannot.
Step 2: The confirmatory mass spectrometry was never done
In the Letby case, the Roche Cobas result was not followed up with mass spectrometry. This is the single most important procedural fact about the insulin evidence. The test that would have confirmed or refuted the forensic inference was not performed. The correct UK forensic laboratory for exogenous-insulin testing — Guildford — was not used. The samples went to the Royal Liverpool clinical laboratory under a clinical-protocol chain of custody.
Step 3: The sample-handling problems
Even if the samples had subsequently been sent for mass spectrometry, the chain of custody and handling would have compromised the result:
- The samples were collected in gel tubes, not the heparinised red-top tubes required for forensic use.
- Centrifugation was delayed beyond the 30-minute window protocol requires.
- Storage was at ambient temperature before refrigeration.
- No forensic chain-of-custody documentation was maintained.
Any one of these alone is sufficient to disqualify the sample from forensic use. Together, they mean the sample was a clinical sample that was never going to bear forensic weight.
Step 4: The alternative causes of high-insulin-low-C-peptide
The Crown told the jury that high insulin with low C-peptide was explicable only by exogenous administration. In the clinical-biochemistry literature that is not true. The pattern can be produced by:
- Insulin auto-antibodies, transiently acquired from a diabetic mother or chronically present.
- Sepsis.
- Adrenal suppression.
- Liver or kidney disease.
- Specific drug cross-reactivities.
- Repeated dextrose infusions (as were given for hypoglycaemia).
- Immunoassay interference (macro-insulin, heterophilic antibodies).
Before the insulin-administration hypothesis can be adopted, each of these alternatives has to be positively excluded. The trial record does not show that work being done.
Step 5: The physiological-plausibility problem
The numerical insulin value reported — on the order of 4,657 pmol/L — is on the scale of adult attempted-suicide cases involving injection of 200+ units of insulin. Prof. Geoff Chase’s physiological modelling (see his biography) shows this value is implausible on the Crown’s own theory of a 0.6 ml spike into a slow-running TPN bag. The numbers do not model to the proposed mechanism.
Step 6: The TPN-bag-retention problem
The Crown’s mechanism required Letby to have added insulin to a TPN bag in the ward fridge, from which multiple nurses drew. No TPN bags were retained for forensic chemistry. There are therefore no physical exhibits for the insulin allegation. The theory rests entirely on the inference from the Roche screening result.
Step 7: The laboratory-protocol change
The Royal Liverpool laboratory’s own protocol changed in 2012 to state explicitly that the laboratory could not diagnose exogenous insulin. The 2010 protocol — in force at the time of the Letby sample testing — did not include that statement. This means the laboratory itself has acknowledged, in its post-2012 protocol, that results of the type produced in the Letby case cannot be used to diagnose exogenous insulin administration.
Step 8: What the Joint Insulin Report concludes
The Joint Expert Witness Insulin Report on Babies F and L (May 2025) walks through each of these problems in technical detail. Its conclusion: the insulin evidence in Babies F and L cannot support a criminal finding of exogenous insulin administration. See our summary.
Step 9: What this means for the indictment
If the Baby F insulin count fails, the insulin counts on the indictment fail. That does not itself dispose of all the other counts, but it removes the one count that had a concrete laboratory measurement. What remains — air-embolism-pattern counts, air-in-stomach counts, the trauma counts on the triplet brothers — are all interpretive. None has the concrete-measurement status the insulin count was given.
The post-conviction expert consensus is that the insulin count fails. It is one of the cleanest strands of the CCRC review.