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The Insulin ‘Evidence’ in the Lucy Letby Trial. Criminal Justice in England: Disagreeable Facts

Updated: Nov 16, 2023

Benjamin Myers, KC, was the barrister representing Lucy Letby during her trial. Prior to the trial Mr Myers appeared to have made a serious error that likely went some significant way to securing Ms Letby’s conviction.

Mr Myers accepted the prosecution‘s claim that insulin had been added to both the Babiven parenteral nutrition (PN) bags and the dextrose bags, intended for the neonates on the ward, who were unable to take nutrients by mouth.  This claim is what the courts in England call an 'agreed fact.' This is where the two sides agree that the evidence being submitted is unquestioned, and is therefore true. This means that despite there being no logical reason for Mr Myers to concede to the claim that insulin was added to Babiven and dextrose solutions, he did anyway. There was no evidence that the neonates under Ms Letby’s care had been improperly given insulin, by her or any other person.

Mr Myers accepted this agreed fact, along with the prosecution, Nicholas Johnson, KC.  In both cases, it is important to state that neither of these individuals are medical professionals, nor do they have a scientific background.  Rather, the expert witnesses would have advised these individuals that the two blood tests for insulin, which were discovered by Dr Brearey, in 2018, definitively proved there was a poisoner on the ward.  This claim was tolerated owing to both barristers having a poor understanding of the testing that was used to determine the insulin concentration in the blood.  In addition, there are a number of key facts which further reveal why the claims surrounding the insulin poisoning are erroneous and should never have been tolerated in this case.

  1. Lack of direct evidence There is no direct evidence from the bags of Babiven or dextrose solutions that insulin was added to the bags.

  2. The insulin concentration exceeded the bounds of reason The test that was performed to determine the insulin concentration provides physiologically impossible readings, that are in the order of 10 - 70 times higher than what would be expected in a premature neonate.

  3. The insulin concentrations would be lethal An adult with a blood concentration of insulin of 4657 pmol/L would be in a hypoglycemic coma for days, and would be unlikely to survive. Kernbach-Wighton and Puschel performed a retrospective review of 12 cases of suicide by insulin, with a postmortem time frame of one day to six weeks. The authors found that peripheral blood insulin concentrations were elevated (range 1-155 μU/mL; mean 49 μU/mL). When converting µU/ml to pmol/L this results in a maximum concentration of 930 pmol/L of insulin. (Kernbach-Wighton, et al., 1999)

  4. Unreliable blood tests For one neonate, the blood taken at 6pm was used to test for insulin, c-peptide, and glucose.  This test showed that the insulin concentration was 4657/pmol/L, the C-peptide was 169 pmol/L and the glucose was 1.3 mmol/L.  Witihin two hours of the blood test the baby’s blood sugar level was ~4 mmol/L, which is normal.  There is no way that the blood sugar level would return so quickly given the extremely high insulin concentration.

  5. Test Exceeds the Cut off Most tests for insulin have a cut off below 4657 pmol/L making it highly likely that was not even a valid test.

  6. No replication of tests None of the samples were retested.  If a medical doctor observed such extremely high blood insulin levels they would be required to test the bloods again to make sure they were within a normal range.

  7. No distinction between exogenous and endogenous insulin The blood test for the insulin uses an antibody that binds endogenous and exogenous insulin. There is no way of determining whether there is exogenous or endogenous insulin being detected.

  8. Cross-reactivity with other proteins The precursor to insulin is a protein called proinsulin.  Proinsulin shares sequence similarity with insulin , but has no ability to affect blood sugar levels. If proinsulin were very high it would be detected as insulin in the assay.

  9. Test reliability is questionable The tests used to detect insulin are inherently unreliable, and different assays will yield completely different results.  This means that it is necessary to perform multiple tests over a short period of time.

  10. In short, there is nothing definitive that shows the insulin they measured is exogenous.

Though most devastating of all, during the trial the witness Dr Anna Milan, testified as to the relevance of the insulin blood test data. Dr Milan permitted test data that falls outside the range of anything physiologically possible to be assessed as valid and even went so far as to state:

‘the only way you get a pattern like this is if insulin is given to a patient.’

This statement is wholly erroneous as confirmed by a statement on the website of the Liverpool testing facility which states:

"Please note that the insulin assay performed at RLUH is not suitable for the investigation of factitious hypoglycaemia. If exogenous insulin administration is suspected as the cause of hypoglycaemia, please inform the laboratory so that the sample can be referred externally for analysis."

The Barrister, Benjamin Myers, KC, conceded that someone poisoned the Babiven and dextrose solutions with insulin. He conceded that on the basis of false claims asserted by a witness.

In April 2023, I sent a document to Mr Myers detailing the myriad issues surrounding the insulin claims in the case, the document was 27 pages long. Mr Myers, and Mr. Thomas (the solicitor), were grateful for the information and confirmed in emails that they would make the information available to their expert witness.  I explained in a later email to Mr Myers that Dr Milan’s claims were contradicted by her own testing facility, still the case continued and both the prosecution and defence maintained that there was actually valid evidence of exogenous insulin, where no such evidence existed.

Further compounding these issues is the fact that a recent study demonstrated that insulin blood tests give wildly different results depending on the manufacturer of the kit (Rosli et al., 2022). This finding is not unknown to most lab scientists, there are a wide array of confounding variables to consider when assessing the validity of a blood test result. The greatest margin of error can occur just through the poor handling and storage of the kit used to carry out the test. This fact should have been known to Dr Milan when she testified that the only cause for such high blood levels was the administration of exogenous insulin. The more likely cause was sample degradation, improper handling/storage of the kit, human error, and poor technique.

A graph of insulin concentration
Figure 1. The graph shows how different concentrations of insulin, from the same blood sample, are obtained due to variations in the manufacturer of the immunoassay kits

(Adapted from: Rosli et al., 2022)

Every lawyer who is representing a person in a criminal case has a duty to protect the procedural rights of their client. Conceding to claims that are not proven, is not doing that, though what it means on a procedural level is a question only a lawyer can answer…. I will leave you with a screen shot of the notice from the testing facility which performed the insulin blood tests.

We must begin to ask the tough questions, and we must challenge these sorts of omissions, and failures in our criminal justice system.

N.B. It was recently identified that as of April 2023, Professor Peter Hindmarsh the one of the main prosecution witnesses who testified regarding the insulin evidence is subject to an interim order by the GMC. This order means that a complaint was submitted against Prof Hindmarsh at some point either prior to or during the period in which he was giving testimony for the prosecution in the Letby case. Crucially, the interim order limits Prof Hindmarsh's practice to work at UCLH and "He must notify any instructing organisation when acting or accepting instructions as an expert witness as to this and any ongoing GMC investigation."

Instructions for insulin immunoassay
Figure 2. Instructions for Insulin Immunoassays used in the Letby Case


Kernbach-Wighton G., Püschel K. On the phenomenology of lethal applications of insulin. Forensic Sci Int. 1998 Apr 22; 93(1): 61–73. 9618911.

Rosli, N, Kwon, H-J, Lim, J, Yoon, YA, Jeong, J-S. Measurement comparability of insulin assays using conventional immunoassay kits. J Clin Lab Anal. 2022; 36:e24521

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