My nan who I care for has been admitted to hospital with a uti her behaviour has been badly affected by the infection, she is delirious . . .
My father was well enough to drive until he was 88, and then it was his, rather than his GP's decision to give up. The first time I found him at all confused was when he had a UTI. The out of hours doctor visited and gave some antibiotics and he recovered his wits within the day. The same thing, bewilderment followed by recovery, happened with subsequent UTIs and also with a chest infection.
Later, he was hospitalised with urisepsis, which is when the bug escapes from the urinary system and causes septicaemia (bacteraemia). May I suggest you ask the hospital doctors what is the bug they're treating your Nan for, and also whether the diagnosis is urisepsis?
On my father's part this episode did mark a point in his decline but he recovered most of the way after leaving hospital. As an in-patient he varied between being confused and disoriented to completely delusional. When he got home he still had some persistent funny ideas but mostly related quite well. He did need a lot of sleep though, either in an arm chair or in bed. It's very tiring being in hospital.
I hope your Nan starts to recover, but being in hospital is disorienting at any age and especially in the elderly. I hope that when she's discharged there is help at first so your Nan may get used to familiar things and places. From my own small experience it'll only be after a small number of weeks you'll be able to tell.
Best wishes,
Fred.