American cyclist Tyler Hamilton is facing the loss of his Olympic gold medal in the cycling time trial because two separate blood tests suggested that he might be guilty of "blood doping." Hamilton, who has up to now enjoyed a squeaky-clean image, denies the charge. What is blood doping? How does it improve an athlete's performance?
"Blood doping" refers to any illicit method of boosting an athlete's red blood-cell supply in advance of competition. The typical adult male's hematocrit—the percentage of his blood that is composed of red blood cells—hovers around 45. Since red blood cells carry oxygen through the bloodstream, increasing the number of them allows an athlete's blood to deliver oxygen to muscles more efficiently, reducing fatigue and giving the athlete an edge. Endurance athletes often train at high altitude for precisely this reason. The lower air pressure and diminished atmospheric oxygen at altitude spur the body to generate extra red blood cells, and can bump the hematocrit up two or three (non-illicit) percentage points.
Athletes can get a bigger—and illegal—boost by injecting themselves with erythropoietin (EPO), a hormone that stimulates RBC production. A urine test for artificial EPO was introduced in 1997, but it's not foolproof; while testable traces of artificial EPO disappear from an athlete's body within four days, the hormone's effects are strongest three weeks after injection.
Hamilton is suspected of using a different method of blood doping, one that requires no hormones; instead, concentrated red blood cells are transfused directly into the bloodstream a week or less before competition. For reasons of convenience and safety, autologous transfusions, in which the cells are the athlete's own, are reportedly far more common than homologous transfusions, in which the cells belong to someone else with compatible blood. Typically, an athlete has up to four units of blood removed a month or more before competition. Technicians then use a centrifuge to separate the red blood cells from this sample; the RBCs are placed in cold storage, only to be reinfused shortly before the big race. (The athlete may even use EPO months before an event to boost the number of RBCs in the units of blood that get removed; the EPO will be undetectable by race day.)
In the past, the only way to test for blood doping was by testing an athlete's blood for an unnaturally high hematocrit; the International Cyclist Union (UCI) bans racers for 15 days if random tests turn up a hematocrit over 50. Hamilton is the first athlete to receive a positive result from a new homologous transfusion test, developed in Australia by Dr. Michael Ashenden's team at Science and Industry Against Blood Doping and introduced at this year's Tour de France. Based on methods used in hospitals to differentiate between maternal and fetal blood in pregnant women, the new test distinguishes among nearly 200 different proteins present in blood cell walls, and can identify blood types far more specific than the A, B, and O on your donor card. Ashenden says his Australian team should have a test for autologous transfusions by 2006.
Though some believe that transfusion is no more immoral than altitude training, the practice is banned by the UCI. And it's dangerous: Cyclist Jesus Monzano says he nearly died after being injected with poorly stored blood in 2003, and an extraordinarily high level of RBCs in the blood can tax athletes' hearts. It's hard work for the organ to push sludgy blood through an athlete's veins.
[Update, Sept. 23: The Associated Press reports today that Hamilton will keep his gold medal; results of a backup test on his Olympic blood sample were deemed inconclusive and the IOC dropped its investigation of Hamilton's case. However, Cycling News reports that Hamilton may still face disciplinary action from the UCI; his blood tested positive for transfusion at another race this month.]
Explainer thanks Mandy Lovett and Dr. Greg Crowther at the University of Washington.