I've seen people around using them past the expiration date and they seem to do fine. With my last vial I found when I got to the real dregs it didn't work well at all - I had to practically double the dose over a few days and still didn't get consistent results. New vial and everything went back to normal. But that was really the bitter end of it, and it was clear from the #s that SOMETHING was different, no question.
Sometimes they will appear to improve on a dose, but then you lose that and the #s go higher and you have to raise the dose again. It seems to have to do with their body adjusting to the insulin - if it isn't enough insulin they might get better #s some of the time, but not consistently. It's kind of mysterious to me, and Jimmy's #s are varied enough to keep the picture confusing. Ultimately though I think you probably have to grit your teeth and go with dose increases every few days until you see consistent good #s. That's what I had to do with Bix and it was tortuous (the occasional good #s he got REALLY threw me off, I kept hoping hoping they would come back on their own, but they never did).
It does seem like they psychically talk to each other sometimes, doesn't it? It's weird how there seem to be waves when a bunch of cats are all doing well, or all throwing wacky #s. Makes ya' wonder, ya know?