This is exactly what I was saying up-thread.
Get a good lot of booze down you, and then the alcohol is in your stomach.
Stomachs don't pass alcohol into your blood very well, but give it quite a while and it'll be in your colon. If you've been eating, the alcohol takes even longer to get to your colon.
Once there, it gets absorbed quite quickly, but it depends on your weight, your age, your race, and your sex, how quickly this happens. Once in your blood stream, your liver will get rid of it, but it depends on how good your liver is how fast it does this.
You need to leave it an hour or two - even more - before you get a good worthwhile result from a test. I know a chap - no names no pack drill - goes down the pub, drinks three (normal strength) beers in about an hour, and then drives home. He's fine I'm sure, but he only drives a couple of miles. Test him as he leaves and he'd be fine. Test him a hour after he gets home, and he'd test positive.
Testing-while-you-booze isn't any good at all.
Mick F. Cornwall