Blood doping
Blood doping is the practice of illicitly boosting the number of red blood cells (RBCs) in the circulation in order to enhance athletic performance. Because they carry oxygen from the lungs to the muscles, more RBCs in the blood can improve an athletes aerobic capacity and stamina.

# Methods
The term blood doping originally meant literally doping with blood, i.e. the transfusion of RBCs. RBCs are uniquely suited to this process because they can be concentrated, frozen and later thawed with little loss of viability or activity. There are two possible types of transfusion: homologous and autologous. In a homologous transfusion, RBCs from a compatible donor are harvested, concentrated and then transfused into the athletes circulation prior to endurance competitions. In an autologous transfusion, the athlete's own RBCs are harvested well in advance of competition and then re-introduced before a critical event.  For some time after the harvesting the athlete may be anemic.
Both types of transfusion can be dangerous because of the risk of infection and the potential toxicity of improperly stored blood.  Homologous transfusions present the additional risks of communication of infectious diseases and the possibility of a transfusion reaction. From a logistical standpoint, either type of transfusion requires the athlete to surreptitiously transport frozen RBCs, thaw and re-infuse them in a non-clinical setting and then dispose of the medical paraphernalia.
In the late 1980s an advance in medicine led to an entirely new form of blood doping involving the hormone erythropoietin (EPO). EPO is a naturally-occurring growth factor that stimulates the formation of RBCs. Recombinant DNA technology made it possible to produce EPO economically on a large scale and it was approved in US and Europe as a pharmaceutical product for the treatment of anemia resulting from renal failure or cancer chemotherapy. Easily injected under the skin, pharmaceutical EPO can boost hematocrit for six weeks or longer. The use of EPO is now believed by many to be widespread in endurance sports.
EPO is also not free of health hazards: excessive use of the hormone can cause polycythemia, a condition where the level of RBCs in the blood is abnormally high.  This  causes the blood to be more viscous than normal, a condition that strains the heart. Some elite athletes who died of heart failureusually during sleep, when heart rate is naturally lowwere found to have unnaturally high RBC concentrations in their blood.

# Testing and enforcement

## General methods
A time-honored approach to the detection of doping is the random and often repeated search of athletes homes and team facilities for evidence of a banned substance or practice. Professional cyclists customarily submit to random drug testing and searches of their homes as an obligation of team membership and participation in the UCI ProTour. In 2004, British cyclist David Millar was stripped of his world time-trial championship after pharmaceutical EPO was found in his possession. Because athletes sometimes inject or infuse non-banned substances such as vitamin B or electrolytes, the possession of syringes or other medical equipment is not necessarily evidence of doping.
It has also been possible to link athletes to blood doping entirely through documentary evidence, even if no banned substance has been found and no athlete has failed a doping test. The Operacion Puerto case is a recent example.
A more modern approach, which has been applied to blood doping with mixed success, is to test the blood or urine of an athlete for evidence of a banned substance or practice.  This approach requires a well-documented chain of custody of the sample and a test method that can be relied upon to be accurate and reproducible.
One strategy has been to regard any "non-negative" or "unusual" result as evidence of blood doping. The Union Cycliste Internationale (UCI), for example, imposes a 15-day suspension from racing on any male athlete found to have a hematocrit above 50% and a hemoglobin concentration above 17 grams per deciliter (g/dL). A few athletes have normally high RBC concentrations, especially if they have polycythemia, which they must demonstrate through a series of consistently high hematocrit and hemoglobin results over an extended period of time. (Hematocrit (HCT) is the fraction of blood cells by volume that are RBCs. A normal HCT is 41-50% in adult men and 36-44% in adult women. Hemoglobin (Hb) is the iron-containing protein that binds oxygen in RBCs. Normal Hb levels are 14-17 g/dL of blood in men and 12-15 g/dL in women.)
A more recent and more sophisticated method of analysis, which has not yet reached the level of an official standard, is to compare the levels of mature and immature RBCs in an athlete's circulation.  If a high number of mature RBCs is not accompanied by a high number of immature RBCs--called reticulocytes--it suggests that the mature RBCs were artificially introduced by transfusion. EPO use can also lead to a similar RBC profile because a preponderance of mature RBCs tends to suppress the formation of reticulocytes. A measure known as the "stimulation index" or "off-score" has been proposed based on an equation involving hemoglobin and reticulocyte concentrations. A normal score is 85-95 and scores over 133 are considered evidence of doping. (The stimulation index is defined as Hb (g/L) minus sixty times the square root of the percentage of RBCs identified as reticulocytes.)
These threshold levels, and their specific numeric values are sources of controversy. Establishment of incorrect threshold values is one way that false positive test results can be produced by a doping control program.

## EPO
Some success has also been realized in applying a specific test to detect EPO use. In 2000 a test developed by scientists at the French national anti-doping laboratory (LNDD) and endorsed by the World Anti-Doping Agency (WADA) was introduced to detect pharmaceutical EPO by distinguishing it from the nearly identical natural hormone normally present in an athletes urine. The test method relies on scientific techniques known as gel electrophoresis and isoelectric focusing. Although the test has been widely applied, especially among cyclists and triathletes, it is highly controversial and its accuracy has been called into question. The principal criticism has been the ability of the test to distinguish pharmaceutical EPO from other proteins that may normally be present in the urine of an athlete after strenuous exercise.
The validity of a doping conviction based on the EPO test method was first challenged successfully by Belgian triathelete Rutger Beke. Beke was suspended from competition for 18 months in March 2005 by the Flemish Disciplinary Commission after a positive urine test for EPO in September 2004. In August 2005 the Commission reversed its decision and exonerated him based on scientific and medical information presented by Beke. He asserted that his sample had become degraded as a result of bacterial contamination and that the substance identified by the laboratory as pharmaceutical EPO was, in fact, an unrelated protein indistinguishable from pharmaceutical EPO in the test method. He claimed, therefore, that the test had produced a false positive result in his case.
In May 2007 Bjarne Riis, Rolf Aldag, Erik Zabel and Brian Holm, all former members of the Telekom cycling team, admitted to using EPO during their cycling careers in the mid 1990s. Riis also relinquished his title as champion of the 1996 Tour de France.

## Transfusions
In the case of detecting blood transfusions, a test for detecting homologous blood transfusions (from a donor to a doping athlete) has been in use since 2000.  The test method is based on a technique known as fluorescent-activated cell sorting. By examining markers on the surface of blood cells, the method can determine whether blood from more than one person is present in an athletes circulation.
The American cyclist Tyler Hamilton failed this test during the 2004 Olympics but was allowed to keep his gold medal because the processing of his sample precluded conducting a second, confirmatory test. He appealed a second positive test for homologous transfusion from the 2004 Vuelta a Espana to the International Court of Arbitration for Sport but his appeal was denied.  Hamilton's lawyers proposed Hamilton may be a genetic chimera or have had a 'vanishing twin' to explain the presence of RBCs from more than one person.  While theoretically possible, these explanations were ruled to be of 'negligible probability'.
At present there is no accepted way of detecting autologous transfusions (that is using the athletes own RBCs) but research is in progress and the World Anti-Doping Agency (WADA) has promised that a test will eventually be introduced. The test method and its introduction date are to be kept secret in order to avoid tipping off doping athletes, though the most likely assay is a measure of 2,3-bisphosphoglycerate (2,3-BPG) levels in red blood cells.  As 2,3-BPG is readily degraded, autologous transfusions will have lower 2,3-BPG levels.  Since 2,3-BPG does not readily diffuse across the cellular membrane, it is extremely difficult to restore 2,3-BPG levels in transfused cells.

## Notable Blood Doping Cases
Tour de France rider Alexander Vinokourov, of the Astana Team, tested positive for two different types of blood, one type from himself, and another type from a compatible donor, various news sources reported on July 24, 2007. Vinokourov was tested after his victory in the 13th stage time trial of the Tour on July 21, 2007. A doping test is not considered to be positive until a second sample is tested to confirm the first. Vinokourov's B sample has tested positive, and he now faces a potential suspension of 2 years and a fine equal to one year's salary.  He also tested positive after stage 15.
Vinokourov's teammate Andrej Kashechkin was also tested positive for
homologous blood doping on August 1st, 2007, just a few days after the conclusion of the 2007 Tour de France his team withdrew from after the revelation that Vinokourov had doped.