I will see if I can find the papers I read that talk about these numbers; at the lower levels they are very poor measures of quantities. It really all depends how much of the enzimes your body produces and how fast it breaks them down. Thus other than "maybe": are they are drinking a lot or a little- is the best they can get to quantify and even that is pretty skewed with no real published results to back that up from what I could find online after a huge amount of time looking.

The PETH numbers do not directly correlate to blood alcohol level in a truly quantitative way. It is just an enzyme that is produced in response to your body processing alcohol (and in my belief some other things that cause these false positives). This number can be higher or lower for different people. Also the enzyme is broken down by different people at different rates (some of the studies said for instance less than 100% functional liver could effect how fast this happens). The number is greatly effected by how well or poorly your body breaks the enzymes down and is highly coupled to time in that regard as well.