Lucy Letby: the insulin smoking gun

The test results showing that insulin had been given to them was the only piece of concrete evidence of criminality throughout the whole of the prosecution’s case. It was the closest thing they had to a smoking gun and became the keystone for their case as a whole.

Johnson’s argument ran that if the jury could agree that Letby had deliberately poisoned two babies, they could also reasonably conclude that she had harmed others using different methods, even if the evidence for those were less concrete. In the event, the insulin cases were the first on which the jury reached a verdict.

—The actual evidence against Lucy Letby, The Times, February 5, 2025

One flash of lightBut no smoking pistol.

—David Bowie, Ashes to Ashes (1979)

Of all the strands of Lucy Letby’s trial, the insulin evidence is the most difficult — in that it is hard for non-specialists to fathom — but also pivotal, as it is the only “smoking gun” in the whole case. Unlike any other piece of evidence, it seems unequivocally to suggest actual malice.

But that is to beg a large question, so let’s back up a bit.

Now I am no kind of expert about anything much, and certainly not glycaemia. Talk of picomoles, immunoassays and C-peptides alternately bores and intimidates me. So we can leave all that to the experts — perhaps with the marginal note that it is so difficult even they don’t seem to be able to agree about it — and talk instead about the test results, in the round, as evidence.

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Start at the very beginning —

As with many of the allegations in this trial, the Cheshire police appear to have started investigating at the end — nearly the end, at any rate: there was a later twist — and then worked backwards. I can’t help wondering how different the outcome might have been had they only started at the beginning.

Their original focus was this:

Scientific tests suggest two children, Baby F and Baby L, were administered “factitious” insulin.

Since they were not prescribed it, and no one owned up to accidentally administering it, this points, presumptively, to malice.

Even without understanding how immunoassays work, we can see this is still not open and shut: it is circumstantial: it requires an inference that dirty work is afoot.

Without better evidence, it weighs on the prosecution’s side.

But there may be better evidence. It may undermine that inference.

For one thing, the test may not have been reliable: perhaps the apparatus was mis-calibrated. Maybe the sample was contaminated. Perhaps the result was a false positive. Maybe the examiner misread it.

But even before doing that, we should first look with an open mind for positively innocent explanations. For, all other things being equal, deliberate insulin poisoning on a neonatal ward is not common. Over the history of healthcare, it has not happened very often.

If the odds are there is a more likely explanation, the least we should do is look for it.

But it doesn’t seem the Cheshire police did this with any great gusto. Rather, armed with their burgeoning theory of an angel of death on the ward — probably with a particular “death angel” in mind — the police saw a smoking gun. They spent little time looking for innocent explanations. They spent little time corroborating this one.

They had their suspect.

This might come to be seen as an oversight. For if there has been an insulin poisoning, we would expect other evidence to corroborate it. We might ask who accessed insulin. We might ask whether any insulin was missing. We might look for tell-tale signs of tampering with drip bags. And we might want some evidence that Ms. Letby had had something to do with it.

Remember the burden of proof: the Crown must show beyond reasonable doubt that this was factitious insulin, and that it was maliciously administered, and that Lucy Letby administered it. The Crown had a prima facie case for this first limb, but nothing to support the second or the third.

Unasked questions