Dui, for ativan ...what are the cut off blood levels to trigger a positive test?

If the test is positive the prosecutor will likely charge, if the test isn't then they won't. For therapeutic levels you can google Winek, he is considered by the prosecution to be a reliable resource. Even if you test positive the prosecution has the burden to demonstrate that the drug impaired your ability to drive. A DUI defense lawyer will likely be able to help you with this.

There is no magic number or "cutoff." Some labs observe a "cutoff" for reporting a positive result, but that's not really the issue anyway. The prosecution must prove the Ativan impaired your driving. I suspect there's more to the story. Get a lawyer.

There is no set amount other than the lowest amount which the lab tests for. The issue is for the prosecution to proof that whatever amount indicated in your test result caused your driving to be impaired. Law enforcement officers know this and are consistent in their testimony to proof that it did ("crossed the center line two times"; "hit the curb"). This is what you need to be prepared to defend.