To determine the level of the drug tacrolimus in your blood in order to establish a dosing regimen, maintain therapeutic levels, and detect toxic levels
As soon as tacrolimus therapy begins, frequently at first, then at regular intervals to monitor concentrations over time
A blood sample drawn from a vein in your arm
The sample should be collected 12 hours after your last dose and immediately prior to your next dose or as directed by your healthcare practitioner.
Tacrolimus, also known as FK506, is an immunosuppressive drug that is given orally or intravenously to people who have had a kidney, liver, heart, or other organ transplant. It is a potent drug that helps to prevent rejection of the transplanted organ by the body. This test measures the amount of tacrolimus in the blood.
Normally, a person's immune system recognizes a new, transplanted organ as foreign and begin to attack it. Tacrolimus limits this response and helps to prevent organ rejection by inhibiting the activation of certain immune cells called T-lymphocytes.
The level of tacrolimus 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 person may experience symptoms associated with toxicity.
Dosages must be tailored to the individual. Often, people will begin with higher doses of tacrolimus at the start of therapy and then decrease the dose over the next few weeks.
Tacrolimus is usually taken twice a day at set intervals before or after meals. When a person takes a dose, the blood concentration rises and peaks within about 2 to 3 hours and then begins to slowly drop. The blood test is usually measured as a "trough level," meaning that sample collection is timed for 12 hours post-dose and/or immediately prior to the next dose, when the drug's level in the blood is at its lowest.
How is it used?
The tacrolimus test is used to measure the amount of the drug in the blood to determine whether the concentration has reached a therapeutic level and is below the toxic level. It is important to monitor levels of tacrolimus for several reasons:
- There is not a good correlation between the dose of tacrolimus given and the level of drug in the blood.
- How individuals absorb and metabolize oral doses of tacrolimus can vary greatly depending on the time of the dose and what, if any, food the person has eaten as well as the genes they have inherited (read Pharmacogenetic Tests).
- Tacrolimus can cause kidney damage (nephrotoxicity), especially in high doses. Measuring levels in people who have had a kidney transplant may help to distinguish between kidney damage due to rejection (because drug level is low) and kidney damage due to tacrolimus toxicity (drug level is high).
- Tacrolimus can increase the risk of developing serious bacterial, viral, fungal, and protozoal infections because it decreases the body's immune response.
When is it ordered?
A tacrolimus test is ordered frequently at the start of therapy, often daily when trying to establish a dosing regimen. Once dosages have been established and shown to be well tolerated, the frequency of tacrolimus testing may be decreased. The test is also performed when dosages are changed and whenever someone has symptoms that suggest side effects, toxicity, or organ rejection.
- Kidney damage (nephrotoxicity)
- Tremors, headache (neurotoxicity)
- High blood pressure
- Nausea and vomiting
- Electrolyte disturbances, such as hyperkalemia
- Intermittent and/or persistent ringing or roaring in the ears (tinnitus)
- Elevated heartbeat
- Blurred vision
Monitoring at intervals is necessary as long as someone is taking tacrolimus.
What does the test result mean?
A concentration that is higher than the established therapeutic range may increase the risk of associated toxicity, including damage to the kidneys and nerves. A concentration that is too low may lead to rejection of the transplanted organ. Side effects may be seen at any dosage but tend to be more severe with a higher tacrolimus level.
The therapeutic range established by a laboratory will depend on both the method used to measure the drug and the type of transplant. Results from different methods are not interchangeable. A healthcare practitioner will usually send samples sent consistently to the same laboratory and will be guided by that laboratory's therapeutic ranges.
Is there anything else I should know?
A variety of drugs can increase or decrease tacrolimus levels in the blood. These include calcium channel blockers, antifungal drugs, macrolide antibiotics (such as erythromycin), protease inhibitors, chloramphenicol, and other immunosuppressants, anticonvulsants, and antimicrobials. Tell your healthcare practitioner about any other medications you take, especially cyclosporine, sirolimus, nelfinavir telaprevir, boceprevir, and amiodarone. Many other prescription drugs, grapefruit juice, and herbal supplements such as St. John's Wort can also interact with tacrolimus.
Topical tacrolimus may be used to treat a variety of skin conditions, such as eczema, that have not responded well to other medications. This topical use of tacrolimus is intended to be local, not systemic, and is not usually monitored.
Tacrolimus increases the risk of developing malignancies such as lymphoma and skin cancer, so sunlight should be avoided when taking this drug. It can also cause new onset diabetes, especially in African Americans and Hispanics so a healthcare practitioner may monitor a person's glucose levels. Tacrolimus may also cause QT prolongation, a condition that affects heart rhythm.
People should not alter their dose or the time that they take their dose without consulting with their healthcare provider. Tacrolimus should be taken consistently with respect to meals. Food, especially high-fat meals, can significantly reduce the bioavailability (amount of the drug that reaches the circulation) compared with fasting.
Women who are pregnant or planning to become pregnant should talk to their healthcare practitioner about associated risks.
How long will I need to be on tacrolimus?
Who orders tacrolimus tests?
Should I tell my other healthcare providers that I am taking tacrolimus?