EPO filled syringe. EPO is a naturally occurring glycoprotein hormone that controls red blood cell (rbc) production (erythropoiesis). It can be used (illegally) by athletes to increase the level of rbcs in their blood, allowing them to transport more oxygen around their body. This test looks for raised levels of rbcs in a sample. If the rbcs constitute more than 50% of the volume of blood it is presumed that the athlete took EPO.