http://www.thorograph.com/archive/files/tdn_drugs4.pdf
the testers in that article who are confident that the absence of positives for particular substance = the absence of doping with that substance need to ponder this:
http://www.tmz.com/2012/05/15/lance-armstrong-doping-interview-gavin-newsom-video/
They also need to ponder Part II of the same series.