20mg/100milliwhatnots means nothing to the alcoholic pilot. You can't even test yourself. He/She just wants to get on the flight, with a hangover, get over the suffering of not having a drink until the next rest period and so it goes on. At last you get home and - BLISS. Two days off with no worries because they have been obliderated. But they always come back eventually. That's the tragedy of the disease.
How many times have you heard of crew-room parties when a room has been trashed, not just through high spirits but because the crew were smashed and groped their way home?
I've been there, seen it, done it (but not the trashing) and got T-Shirts to prove it.
Alcoholism is viscious and totally baffling to those who have never suffered from it. And that includes your AME.