TPN bag chain of custody — the missing physical exhibits
Prosecution claim
The Crown's insulin theory alleged Lucy Letby added insulin to a TPN (total parenteral nutrition) bag in the ward fridge, from which the bag was then hung for Baby F and subsequently Baby L. The physical mechanism required a contaminated TPN bag.
Counter-evidence
No TPN bags were retained for forensic chemistry. There are therefore no physical exhibits for the insulin allegation. The Crown's theory rests entirely on the inference from the Roche Cobas immunoassay blood result. Multiple nurses on the unit drew from the same ward-fridge TPN stock; if a bag had been contaminated, identifying which person had contaminated it on forensic grounds would require the physical bag — which does not exist. This chain-of-custody failure is itself a structural problem with the insulin count: the theory requires a physical act on a physical exhibit, and the physical exhibit was not preserved.
You cannot convict on a theory that requires a physical act on a physical exhibit when you have not kept the physical exhibit. There is no insulin-contaminated bag to examine. There never was.
What the jury heard
The Crown's narrative of insulin-in-TPN-bag was presented as a plausible mechanism consistent with the Roche Cobas blood result. The absence of physical bag evidence was not systematically flagged as a chain-of-custody failure.
What the Panel says
The Joint Insulin Report identifies the chain-of-custody problem as one of multiple reasons the insulin evidence cannot support a criminal finding of exogenous insulin administration.
What independent experts add
- TPN bags on a neonatal unit are prepared by pharmacy in sterile conditions and delivered to the ward.
- Multiple nurses on the unit drew from the same ward-fridge TPN stock.
- If a bag had been contaminated, identifying the person responsible on forensic grounds would require the physical bag.
- No TPN bags were sequestered for forensic chemistry at any stage of the investigation.
- Forensic chain of custody for a physical-act allegation requires the physical exhibit to be preserved.
- The chain-of-custody failure mirrors the sample-handling failures on the Roche Cobas samples (gel tubes, delayed centrifugation, ambient-temperature storage).
- Together, the two failures mean neither the alleged action (TPN contamination) nor the alleged evidence (insulin measurement) was handled to forensic standard.