The Test Sample
What is being tested?
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. This test measures the amount of sirolimus in the blood.
Normally, a person's immune system recognizes a transplanted organ as foreign and begins to attack it. Sirolimus limits this response and helps to prevent organ rejection by inhibiting the activation and production of white blood cells called T-lymphocytes. It also inhibits antibody production.
Sirolimus is typically taken by mouth (orally) and, after ingestion, is absorbed from the digestive tract. The drug's concentration peaks in the blood within a couple of hours and then gradually declines. It is metabolized by the liver and is eliminated from the body in the stool (more than 90%) and urine (less than 2%).
The U.S. Food and Drug Administration (FDA) has currently approved sirolimus for use in people 13 years of age and older who have kidney transplants. It appears to be less toxic to the kidneys than other options and can be given in conjunction with the other immunosuppressant drugs cyclosporine and tacrolimus. Studies to evaluate its use in other age groups and types of organ transplantation, such as liver or lung, are ongoing, but so far, no data have convinced the FDA that sirolimus is appropriate for children under 13. Like other immunosuppressant agents, sirolimus may cause side effects and adverse reactions and is associated with an increased risk of infection and the development of lymphoma and skin cancer.
The sirolimus level must be monitored over time with laboratory tests because the drug has a narrow therapeutic index or range of effective concentration. If the drug concentration is too low, organ rejection may occur; if it is too high, symptoms associated with toxicity may develop. The sirolimus blood test is usually measured as a "trough" level, with blood collection just prior to the next dose – at the drug's lowest concentration in the blood.
Typically, sirolimus is given with cyclosporine and corticosteroids. Dosages must be tailored to the individual, and sirolimus and cyclosporine must both be monitored. Often, people will begin with an initial higher dose (loading dose) of sirolimus and then are tapered to a lower dose. In people considered at low risk for complications, cyclosporine may be decreased or stopped after 2-4 months and sirolimus concentrations increased.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed; however, the sample is typically collected 12 hours after the last dose and immediately prior to the next dose, or as directed by the health practitioner.