How can I be charged with a "drug DUI" when I had used the drugs days before driving and thereby was not "under the influence" when I drove?

Not Currently "Under the Influence"

Pennsylvania's DUI law prohibits a person from driving, operating, or being in actual physical control of a motor vehicle with any amount of a Schedule I, II, or III controlled substance or a metabolite of such substance in his or her blood. While the DUI law prohibits a person from having "any amount" of a controlled substance in the blood, the Department of Health is actually required to prescribe minimum levels of controlled substances that must be found in the blood for the test results to be admissible in a DUI prosecution. The minimum levels of controlled substances required to justify a DUI prosecution are set relatively low so even minor use of drugs can lead to DUI charges. While alcohol is generally expelled from a person's blood and body within hours of consumption, evidence of drug use often remains in one's blood for a much longer period of time. This means that a positive drug test may evidence drug use that occurred days prior to the test, so the person may not be under the influence or actually intoxicated by the drug when the DUI arrest and subsequent testing occur. Under the per se drug DUI law in Pennsylvania, a person can still be prosecuted for DUI if he or she has an amount of controlled substance in his or her blood that exceeds the minimum level established by the Department of Health even if the person was not under the influence of the drug while driving or operating his or her vehicle.