I have worked for the local Law Inforcement as an IT person and I *know* for a fact that (at least in this region of the world):

A) A positive breath test gets you pulled in for a blood test.

B) The devices used by the cops (ie, breath testers, radars, etc) went through rigorous calibrations and are re-calibrated on a regular basis (a workmate spent 3 months on one of these calibration exercises).

While the breath-test will never give a 100% accurate BAC level, it *will* give you an indication of whether the driver should be pulled for further testing, and do *that* with reasonable accuracy.