Referencing this Straight Dope Staff Report.
The report ends by saying: Further, such trace amounts could show up in the person’s next urine drug screen, and a positive for cocaine metabolites in the urine would warrant disciplinary action. There’d be no way for anyone to tell an “on-the-spot drug tester” from a recreational user.
Is this strictly true? I thought that by testing hair from a person’s head they could accurately determine when and how much of a particular drug was used over the whole period of the hair’s growth?