Armstrong denied the charges in a statement: “I have never doped, and, unlike many of my accusers, I have competed as an endurance athlete for 25 years with no spike in performance, passed more than 500 drug tests and never failed one.”
http://www.reuters.com/article/2012/06/13/us-usa-armstrong-doping-idUSBRE85C1IN20120613
Not defending Armstrong but if the above is true then he has been damn lucky in the tests or the charges are just that, charges without proof.
Drug Tests can be fraudulently passed.
[B]lood samples from Armstrong in 2009 and 2010 that were “fully consistent with blood manipulation including EPO use and/or blood transfusions.”