There are numerous ways to deliver hormones to the bloodstream and hence, the urine ... to be later detected by laboratory tests.
One way is to maliciously mix the banned isotope or other substance with water and "Splash" it on the rider as he or she passes close to "Spectators" (Well meaning tour followers often attempt to aid riders in many ways). Skin absorbtion does the rest and the urine sample will test positive as the test sensitivity is extremely high.
I'm surprised this fact hasn't been raised. It's important becuase Landis can never prove a negative (That he didn't take drugs).
All he can do is (1) establish doubt that he intentionally administered the drugs that were found or (2) that the tests or testers were in error and there were no drugs present.