I've never used interrupts.... And I don't know if you can set an analog threshold to trigger an interrupt... But are you sure you need one? What are you interrupting?
From what you've said, it's just reading continuously (or periodically) in a loop and if "something happens" you need "do something" (transmit a message). I don't see anything else going-on that you need to interrupt or anything time-sensitive where you'd have to interrupt whatever is going-on.