In the event that she is able to answer that question again, Letby will be able to point out that the tests did not demonstrate that the infants were poisoned. But that isn't her job!
I get that it's not her job to disprove the prosecution's case, but it was her job to accept or dispute the evidence presented.
Given that her argument was that no-one deliberately hurt the babies and that it was just coincidence that she was suspected of doing it, I just can't fathom her being innocent but also accepting that someone else poisoned babies on two of her shifts.
I realise she isn't an expert on those tests, but if she's innocent, then logically, how did that evidence that she accepted as a fact square with what she says actually happened? She obviously knew about that evidence in advance of the trial, so how did she think that would fit in with her version of events if she accepted it?
I can see why, if she were guilty and had added the insulin and then they presented evidence that insulin had been added, her best course of action would be to accept the evidence that insulin was added (as she would have no reason to doubt the accuracy of the tests as she knew did it) but deny it was her. But why accept evidence of an attempted murderer on the unit if she didn't do it and genuinely believes there is no-one deliberately harming babies?
Now that it's agreed evidence, it would presumably stand as such for any appeal or retrial anyway, so I doubt she'd get a chance to try answering that question with a different answer again.