At a Glance
Why Get Tested?
To determine the concentration of sirolimus in the blood in order to establish a dosing regimen, maintain therapeutic levels, and detect toxic levels
When to Get Tested?
Early in the course of sirolimus therapy and whenever the dose is changed; frequently at first, then at regular intervals to monitor concentrations over time; whenever excess or deficient levels are suspected
A blood sample drawn from a vein in your arm
The Test Sample
What is being tested?
This test measures the amount of sirolimus in the blood. Sirolimus is a relatively new immunosuppressive drug that is given orally to patients who have had an organ transplant. Sirolimus is a compound produced by the bacteria Streptomyces hygroscopicus. It was originally discovered in a soil sample from Rapa Nui (Easter Island), about 3,200 kilometres east of Chile in the South Pacific. When ingested, sirolimus is absorbed from the gastrointestinal tract. Concentrations peak in the blood within a couple of hours and then gradually decline. Only a small percentage of the dose taken is bioavailable - present in a form that the body can use. Sirolimus has a half-life of about 60 hours. It is metabolised by the liver into several metabolites and is excreted from the body in the stool (more than 90%) and urine (less than 2%).
Sirolimus is currently under the Commonwealth/State Highly Specialised Drugs Program and approved for use in kidney transplant rejection. It appears to be less toxic to the kidneys than some other options and can be given in conjunction with the other immunosuppressive drugs, cyclosporin and tacrolimus. There are insufficient data regarding its use in children and the elderly, and there are cautions against its use in pregnant and breastfeeding women. Like other immunosuppressive agents, sirolimus may cause side effects and adverse reactions and is associated with an increased risk of infection and the development of lymphoma.
Normally, a person's immune system recognises a transplanted kidney as foreign and begins to attack it. Sirolimus limits this response and helps to prevent organ rejection by inhibiting T-lymphocyte activation and proliferation and antibody production.
Sirolimus levels in the blood must be maintained within a narrow therapeutic range. If the concentration is too low, organ rejection may occur; if it is too high, then the patient may have symptoms associated with toxicity.
Typically, sirolimus is given with cyclosporin and corticosteroids. Dosages must be tailored to the individual, and sirolimus and cyclosporin must be monitored. Often, patients will begin with a loading dose of sirolimus and then tapered to a lower dose. In patients considered at low risk for complications, cyclosporin may be weaned after 2-4 months and sirolimus concentrations increased.
Sirolimus is usually taken once a day at set intervals, consistently with or without food. If cyclosporin is also being taken, then the dosages of both drugs should be separated by several hours. 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.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.