You can't siphon off your hemoglobin............but you can transfuse/store your red blood cells and then reinfuse them later.....called autofusion, common practice these days prior to elective surgery with the AIDS fears and hepatitis fears. Unfortunately, the storage time for packed RBCs isn't terribly long, so when EPO's development for cancer patients came about (an artificial stimulant for causing the body to overproduce RBCs within the body), aerobic-challenged athletes began to use it to produce an overabundance of RBCs in the body and along with it a higher oxygen-carrying ability of the blood. (There is a medical condition that is a problem for some persons of Mediterranian descent......thalassemia....an overabundance of RBCs to the pint the blood's viscosity is so high that proper circulation is impared. The "cure" is siphoning off a pint every couple of weeks or so for life.)
The ICU, among other sports bodies, finally developed and continue to develop tests to check for metabolites of EPO and/or its aftereffects on the body. The ICU, for example, limits how high one's hematocrit climbs over a prolonged bike race like the Tour. The hematocrit is a ratio of the solids/liquid of the blood represented as a percentage...........the higher the percentage, the higher the concentration of solids which would naturally include RBCs and their associated hemaglobin.
I believe the upper limit of HCT concentration allowed during the Tour is 55%. While all the cyclists do have their HCTs climb over the three weeks due to continuous dehydration/rehydration cycles, cycles that usually leave them a little more dehydrated each day, climbing over the magic number will disqualify you.
Lance's blood never has spiked higher than the limit nor has his bodily fluids ever been found to have a trace of metabolites of doping, no matter the test performed or when it was done on any sample. If he does, he and his doc are good. And if you subscribe to the theory that "He must because everyone does," then his doping just creates a level playing field for him.