Though testing for the blood-doping agent erythropoietin (EPO) remains in its infant stages, a view into the prevalence of the drug on North America's backstretches is beginning to come into focus.Since a test to identify EPO antibodies was introduced to racing jurisdictions late last year, horses have tested positive in Texas, New York, and Louisiana. This week it was revealed tests on 400 samples taken from Fair Grounds and Delta Downs in February produced 12 positives for EPO antibodies.Dr. Steve Bradley of the Louisiana Sate University Equine Clinic said while a 3% positive rate for EPO in Louisiana may not sound high, it is not possible to truly gauge the pervasiveness of EPO use because of how quickly it exits a horse's system. Trainers and veterinarians are able to halt the administration of the drug in time for race-day samples to be left without a trace. "What's going to be required is off race-day testing," Bradley said . "You don't need to do it to every horse. If they have probable cause from the antibodies that certain people may be using this drug, that will put them in a pool for further monitoring and testing."Positive tests do not currently lead to disciplinary action against trainers and veterinarians because there is not a confirming mechanism to decipher false positives, Bradley said.For now, barns associated with positive EPO tests will have to deal with being more closely investigated and monitored. Bradley compared testing for EPO, a Class 2 drug in the Association of Racing Commissioner's International Classification Guidelines for Foreign Substances, with that of early testing for clenbuterol. He said clenbuterol initially could be picked up in samples, but a confirming method had not been developed to allow for further action."That's what everyone is waiting for [with EPO]," said Barker, who believes a combination of tests currently available could eventually be the key. Barker said it appears through early testing the form of choice on the backstretch is the human form of EPO, which is used as a therapeutic medication for chemotherapy. EPO, a naturally occurring hormone in humans and equines, regulates the formation of oxygen-carrying red blood cells.Cyclists and track and field athletes were the first in the sporting world to use EPO to stimulate red blood cell production for increased oxygen intake. "In the very large spleen of a horse [EPO] can give great oxygen carrying capacity," Barker said. He added EPO is also helpful in clearing lactic acid in the muscles of a horse, which reduces soreness.But in addition to being illegal for its performance-enhancing qualities, EPO abuse has proven to be fatal. Barker said horses "have literally dropped dead" after using EPO because of two major reactions. "When this protein is administered, they see it as being a foreign substance and mount an immune response," Barker said. "In some it is so severe that they literally drop dead. "Horses also develop anemia because they have an immune response from their own EPO, so you have to continue to administer to such animals because there is no longer any red blood cell production. Once there is withdrawal, it could be severe."Barker added it remains to be seen what will be discovered once a complete EPO test is introduced."We're just now being able to find out because we've really had no tests [for EPO[," Barker said. "With some of the other drugs, once a test was developed, some came back without any positives, while others were found to be 10% positive. If the Louisiana cases are any indication, it's in the 3% range. That's not very much, but it's still too many."