I think maybe you misunderstood @wolco's point? I think what they are saying is that, for example, you might do all of your testing on college age males and find that it doesn't give any false positives for people who smoked yesterday but not today. But you still need to test it on 60 year old women to make sure it doesn't give false positives there. In other words, you need to test it for varying lengths of time and also to make sure that it has consistently acceptable results by race, sex, weight, age, etc.