The routine with my company is quite simple. Your line manager administers the test. A zero reading and you just get on with the job. A positive breath test under the limit could (depending on the level) initiate a full medscreen urine test and being taken off the job until results are known.
Fail a test then you are down the road at once, no questions. Also there would be little chance of ever being employed again as a driver.
As for the stress involved, it has become a routine thing. No big deal.
Any incident on the line will result in a medscreen if you are suspected of any fault. After a fatality (suicide) some years ago I was breathalysed three times in the space of 90 minutes. Civilian police, transport police and employer.
As I said before, 36 hrs between glass and seat.