Yes, it would be possible to do this using serial dilutions, but it would have to be someone who understood the test very well, and planned the contamination carefully. But why did only some of LA’s samples test positive (6/17, and according to Ashenden, a couple of others were in the gray area, almost certainly positive, but technically would have been classified as non-positive)? If someone knew the identity of LA’s samples, never mind how, why were only some contaminated?
Some of the samples from other riders were also positive, so it might be argued that someone randomly contaminated the samples, assuming some would turn out to be LA’s. Never mind how this person knew that Ressiott would later expose these results. The problem is that a much higher % of LA’s samples tested positive (35%) than the remaining samples (10-15%, IIRC). So a random selection of samples to be contaminated would be unlikely to produce the results.
Someone, I think it was USPostal, claimed there was evidence that an enzyme in urine could convert endogenous EPO into a substance running on electrophoresis like exogenous EPO. I believe you are referring to a couple of athletes who initially tested positive, but later got off by successfully arguing that this is what happened. It has nothing to do with frozen storage (which inhibits enzyme activity, but can occur in the urine sample as soon as it is given, or even in the body. But this kind of physiology is apparently quite rare, found in only a few individuals. If LA were one of these rare individuals, why hasn’t this problem cropped up before, i.e., why hasn’t he tested positive for EPO in sanctionable circumstances? Why in fact wouldn’t the A samples from ’99 test positive?
The claim by someone that the positive samples showed just the right % of certain isoforms over time during the TDF is BS. There is no relationship between the amount of exogenous EPO in the urine and the % of the isoforms which indicate a positive. One can have a huge amount of exogenous EPO present and barely test positive, and conversely, one can have a barely detectable amount of EPO present and have a very clear positive by isoform proportion. There has to be a minimum amount of EPO present in the urine to proceed to the electrophoretic stage, but at that point, any result is possible.