Choosing between false positive and false negatives has a lot to do with the consequences. If you miss a drug dealer on a screening test, you missed the chance to catch the bad guy. Depending on the drug and social dynamics, this could be fairly benign (pot) or lethal (cocaine, meth, benzos, opioids). You've also communicated to the bad guy that he can get away with it. Which, given what we know of male behaviour, suggests he will push more drug more brazenly next time.