Lucy Letby: the insulin smoking gun

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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 light
But 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.

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

Only very small volumes of insulin needed to be added to the half litre bags of feed; it was not noticeable in the bag, nor would it be apparent that any was missing from the insulin bottle.

—Sentencing Remarks, Mr Justice Goss, August 21, 2023

Now: absence of evidence is not evidence of absence, sure. But when we find absence after absence of evidence where, on a random roll of the dice, once in a while you would expect to find some — if, on every roll, the evidential dice keep coming up snake-eye — we should be asking questions.

No matter how brilliant a criminal mastermind, a killer would have to get some insulin from somewhere, and insert it in the feed bags somehow. All other things being equal, you would expect evidence for these necessary steps to accumulate. The more consecutive absences there are, the more this begins to feel like improbability.

And even if we find none — a successful villain covers her tracks, after all — we should at least expect evidence to point away from other innocent explanations. So let us see:

Was there evidence of Ms. Letby taking or requesting any insulin?

No.

There are no documents, witnesses, or physical evidence (even consistent with Ms. Letby accessing insulin. The prosecution implies she could have taken it opportunistically from the unit’s supply — and logically, this is true, but the fact remains: there is no evidence that she did. Absence of evidence is not evidence of anything.

Is there evidence any insulin went missing?

No.

Stock checks weren’t detailed enough to confirm missing insulin, but it is still true that no discrepancies were recorded. The Crown might say the amounts involved were small enough to go unnoticed — and logically, this is true, but the fact remains: no tangible loss was recorded. Absence of evidence is not evidence of anything.

Is there evidence of Lucy Letby tampering with any feeding bags?

No.

There were no eye-witnesses, and no forensic evidence on any of the bags linking her to any tampering with it. No fingerprints, no puncture wounds, no traces of insulin found where they should not have been. Modern feed bags have sterile ports through which one can introduce medicines, so there was no need to puncture the bags, and the Crown might say the bags were thrown out before they could be checked — and logically, this is true, but the fact remains: no-one saw any reason to check them. Absence of evidence is not evidence of anything.

Is there evidence any feeding bags were tampered with at all?

No.

The bags were discarded as part of routine practice without being checked. The prosecution’s theory assumes they were tampered with — and logically, it is true, they could have been, but the fact remains: there is no evidence that they were. Absence of evidence is not evidence of anything.

Is there evidence Ms. Letby researched insulin, or insulin poisoning, on the internet?

No.

And the police looked. They searched her devices and home. They dug up her garden. Ms. Letby might have been a zen master of surreptition in every other dimension of her criminal enterprise, but she was incautious about her internet search histories. Yet the police found no searches about insulin — nor for that matter, air embolus, overfeeding by milk or liver damage by Vulcan mind-meld. The prosecution might argue that, being a diligent nurse, she already know this anyway, but still. Absence of evidence is not evidence of anything.

Not only was no such evidence uncovered, as far as we know, no one really even looked for it. The police had their smoking gun, their expert witness was on his way from wales, so poor old “Nurse Death” was for the high-jump.

This consistent absence of evidence in scenarios where, on another day, evidence might exist, start to resemble a different kind of smoking gun.

And that is before we consider the twist.

That twist

Imentioned at the top that the police started their investigation near where is should have ended. This where they found their smoking gun: readouts from the insulin immunoassays that raised what they took to be the near certain presumption of foul play. Others would later disagree, but that is where they were. And all experts agree the readings, however reliable, were extremely high. This is how they could be so confident of bringing charges.

They therefore suggest an unfakeable, unhideable piece of corroborating evidence: a dose this huge would carry a very high — not certain, but high — likelihood of rapid coma and death.

So here is the twist, which happened just after the point at which the police drew all their conclusions:

Both infants survived.

Now — we are in the world of probabilities here, so survival is, of course possible, but it was not the most probable outcome if the test results were correct. Here is another missing corroboration: the severe outcomes we would expect from deliberate poisoning have not come about.

If this is a malicious — that’s unusual enough in itself — then it its outome was even stranger.

There is a counter-argument: well, the specialists at the Countess of Chester Hospital were like a crack squad of experts and it was only their swift intervention that saved the day — ok: let’s go with that one. This is something you could expect a master manipulator and criminal expert to know, in which case wasn’t plotting to murder with slow-release insulin in a neonatal ICU, monitored up the wazoo, like a dumb way of trying to kill?

We are in a Condorcet paradox of sorts where each proposition undermines the next in a sort of circular fashion. The Crown can advance three alternatives, but they conflict:

1. Ms. Letby set out to murder infants with lethal doses of slow-release insulin in a Neonatal ICU. She only didn’t get caught because, being a master criminal, she covered her tracks so expertly none of the ICU’s controls or monitoring systems could catch her.
2. Even though she administered skillfully calculated fatal doses, the ICU’s controls and monitoring systems were so good they saved the infants.
3. A master criminal would know how good the ICU was at catching hypoglycaemic infants, so would know not to try murdering by slow-release insulin poisoning.

In any case, isn’t attempting to kill by administering slow-release insulin and then walking away just a bit Batman villain?

Selection bias?

Now there is another implication of the children’s recoveries. It calls into question how, and why, the prosecution select these events for charges in the first place.

An episode of hypoglycaemia followed by a recovery, is not an inherently suspicious event in a neonatal unit. In fact, premature infants are somewhat prone to it. So much so that you would not expect such an episode to be reported on datix, or registered as a “near miss”, or anything like that. It might be recorded, but it would merge into the background noise of the hospital data. There is a lot of data flying around a hospital.

And so it seems: when they came back from the lab the immunoassay results were logged but no further action was taken. They did not pitch up as evidence of “foul play” until the police investigation was well underway. At this point Ms. Letby was already, well and truly, in the cross hairs.

So what happened? Did prosecutors stumble randomly on these records, pull them up, get hinky about them, and only later realise that, by crikey, Ms. Letby was on duty?

Or did it happen the other way around? Did they isolate all Ms. Letby’s shifts, and drill into them looking for anything they could find?

This might seem a small point, but in the context of the “dodgy shift rota” — the only piece of identification evidence positively implicating Ms. Letby — it is massive.

Disconfirmation bias

We are pattern-matching machines. When we are artificially motivated to find certain patterns we get good at zeroing in on them. This is a form of remunerated confirmation bias. We are primed and motivated to see patterns that will promote our own personal significance. Police officers exist to see crime patterns. Prosecutors exist to see, and deliver, convictions. Expert witnesses optimize their fees seeing what their clients want them to see.

A way of disarming this confirmation bias is to seek out gaps in corroborating evidence: things we would expect to find, if our hypothesis were true but which we do not find. To introduce some disconfirmation bias.

The rules of court procedure positively discourage that sort of behaviour, though. Under disclosure rules, any falsifying evidence you do find you must turn over to the defence . This hardly encourages prosecution teams to open-minded enquiry.

Now absence of evidence is not evidence of absence, of course, but all the same, if your theory is that a burglar was in the house, a normally excitable dog who did not bark might be a useful corrective, at least to paint a picture of the side of that Texan barn without the target painted on it.

Concession that the insulin was deliberate

The Court reporting suggests this was less emphatic than it has been made out to be:

Letby is asked if Child E was poisoned with insulin.
“Yes I agree that he had insulin.”
“Do you believe that somebody gave it to him unlawfully?”
“Yes.”
“Do you believe that someone targeted him?”
“No.”
“It was a random act?”
“Yes ... I don’t know where the insulin came from.”
“Do you agree [Child L] was poisoned with insulin?”
“From the blood results, yes.”
“Do you agree that someone targeted him specifically?”
“No...I don’t know how the insulin got there.”
Letby adds: “I don’t believe that any member of staff on the unit would make a mistake in giving insulin.”

See also

• Texas sharpshooter
• Confirmation bias
• Absence of evidence is not evidence of absence