Sirolimus (rapamycin) is an mTOR (mechanistic target of rapamycin) inhibitor used as an immunosuppressant primarily in solid organ transplantation and certain oncologic and rare disease indications. Therapeutic drug monitoring (TDM) of whole-blood trough concentrations is essential due to sirolimus's narrow therapeutic index, significant pharmacokinetic variability, and extensive drug-drug interactions. Whole blood is the preferred matrix because sirolimus partitions extensively into erythrocytes. Sirolimus binds intracellularly to FKBP-12 (FK506-binding protein), and the resulting complex inhibits mTORC1, thereby suppressing cytokine-driven T-lymphocyte proliferation and blocking the cell cycle at the G1-to-S phase transition. Unlike calcineurin inhibitors, sirolimus does not inhibit calcineurin or impair early T-cell activation, giving it a distinct nephroprotective profile but also unique toxicities. It is metabolized primarily by CYP3A4 and P-glycoprotein in the gut and liver, resulting in a long half-life of approximately 60 hours and substantial inter- and intra-patient variability.
Sirolimus (brand name Rapamune) is a medication that suppresses your immune system to help prevent organ rejection after a transplant or to treat certain rare conditions. This blood test measures how much sirolimus is in your blood, usually checked just before your next dose (called a 'trough level'). Keeping the level within the right range is important—too much can cause side effects like low blood counts, high cholesterol, or lung problems, while too little may not adequately protect your transplanted organ. Your doctor uses this result along with your overall health and other medications to adjust your dose. Regular monitoring is a routine and important part of your care.
When elevated: Elevated sirolimus trough levels increase the risk of myelosuppression (thrombocytopenia, leukopenia), hyperlipidemia, proteinuria, impaired wound healing, interstitial pneumonitis, peripheral edema, and opportunistic infections; dose reduction or temporary discontinuation may be warranted. When low: Subtherapeutic sirolimus levels in transplant recipients raise concern for inadequate immunosuppression and increased risk of acute or chronic allograft rejection; dose adjustment, assessment of adherence, and evaluation for drug interactions should be considered.
Sirolimus is not relevant to athletes or the general healthy population. This test is exclusively for transplant recipients, certain cancer patients, and individuals with rare diseases requiring mTOR inhibition—not for performance, recovery, or health optimization in athletes.
Turnaround Time
6 days (up to 10 days)
Fasting Required
No
Order online
Choose your lab and check out. We send your lab requisition automatically — no doctor visit needed.
Get your sample collected
Visit a lab service center near you for a quick blood draw (or book at-home phlebotomy where available).
See your results
Your results land in your Insider portal, ready to review and act on — that easy.
Your price
$50.00$150
3.0× less than retail
Versus the typical direct-to-consumer retail price for this test (illustrative — consumer prices vary by provider and region).
$50.00
$50.00
$50.00
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