The wrong test was carried out to provide evidence of exogenous insulin being administered. This wasn't really challenged and I'm afraid it's the ignorance of some of the people involved in the criminal case and in the unit who did not know or understand this. This is the reason that it wasn't acknowledged and Lucy Letby agreed with it. It's a very specific test that needs to be carried out and it wasn't ordered or performed. They used different test which has high failure rate and cannot distinguish between insulin made by the body and synthetic insulin.
In addition, no insulin was found to have been tampered with or gone missing on the unit. There is some query about whether she would have had opportunity to administer it to a feed bag.
By the time she was arrested she had been under investigation for a very long time and on antidepressants. Admittedly, I have not watched the documentary, but I can understand perhaps that she was resigned to being found guilty no matter what happened.
No one can say with 100% certainty whether she was guilty or innocent but the evidence that she was convicted on is very very poor.
There is however plenty of evidence that that hospital unit was run in a terrible manner. Had high rates of maternal death compared with the average. She certainly cannot being blamed for the deaths of those mothers.
If anybody cares to they should watch the press conference run by Professor Shoo Lee. It s a depressing hour and a half of a catalogue of failings by a facility meant to care for babies.