A Sirolimus Blood Test is a test to measure the levels of sirolimus in blood. Sirolimus, also called rapamycin, is a drug that is used to suppress the immune system in people who have had kidney transplants. It is given in combination with other drugs to prevent rejection of transplanted kidneys. Normally, a persons immune system recognizes a transplanted organ as foreign and begins to attack it. However, Sirolimus limits this response and helps to prevent organ rejection by prohibiting activation and production of white blood cells called T-lymphocytes. It also prohibits antibody production.