Blood doping or bad luck?

Tyler Hamilton, a professional bicycle racer, is appealing his suspension for blood doping with the claim that some of his blood cells are not his own. Specifically he is claiming he is a chimera, with some blood cells from a "twin" who didn't survive to birth.

While he may be right, I doubt that is why he got caught blood doping.

"The presence within one individual of a small population of cells from another genetically distinct individual is referred to as microchimerism" according to a review in JAMA last year (subscription only).

According to the NYTimes article the majority of people are chimeras, maybe everyone. In most people this is due to rare maternal cells that crossed into the fetal bloodstream during gestation and have remained (with or without propagating). Many women also have cells from the children they've borne.

The significance of these cells are unclear. There is some data, for the most part preliminary, that these "foreign" cells may play a role in both autoimmune disease and rejection of transplants. For example, women with the autoimmune disease systemic scleroiss have evidence of more cells from their children than do healthy women. Of course, this doesn't prove causation. Perhaps there is something about inflammation in generaly that causes chimeric cells to expand.

Back to Hamilton. According to the Times article:

Dr. Brown, the only outside scientist that the anti-doping agency suggested to present its point of view, said that blood banks almost never found chimeras. The blood banks used a less sensitive test, but Dr. Brown testified that he himself, using the more sensitive flow cytometry on at least 20,000 blood samples, never found a chimera.

And, he said, reports of people with small amounts of foreign cells do not signify that an athlete with a second population of blood cells had someone else's blood stem cells in his bone marrow. Moreover, he said, Mr. Hamilton tested negative a few months after his positive test last fall. That is consistent with an athlete who had transfusions, was caught, and then stopped.

In addition, Dr. Brown said, another rider on Mr. Hamilton's team, Santiago Perez, also tested positive. (He did not show up for his hearing and was pronounced guilty in absentia.)

"It seems inconceivable to me that there would be two people who were rare chimeras on the same cycling team," Dr. Brown wrote by e-mail.

Of course, Perez could be guilty and Hamilton innocent, although if one team member was blood doping you'd suspect the other was. If they really did test it on 20,000 blood donors and not find any chimeras that would very strongly suggest that Hamilton is guilty. But, as the article goes on to say, others have raised questions about the test, particularly how it is applied in different labs.

Reading the article carefully you note that Hamilton did not raise the possibility that he is a chimera from his mother. My best guess is that he is guilty and knows the transfused blood came from someone other than her, so that defense could be proven wrong.

If he really is innocent, which I hope he is, but doubt, he could make a pretty strong case by showing he consistently showed evidence of chimerism using very sensitive techniques (the technique that was used to detect foreign cells initially, then not in follow up may not be sensitive enough to show chimerism in every sample).

If he could show that the chimeric cells were always the same and matched the original sample, he could argue he really was a chimera. Showing the cells could plausibly come from a vanishing twin (based on his and his parents genes) would strengthen the argument, particularly if they didn't match any living sibling (who could be the source of the blood for doping, if that is what happened).

UPDATE: fixed punctuation