Tacrolimus 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. Dosages must be tailored to the individual. Often, patients 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, blood concentrations rise and peak within about 2 to 3 hours and then begin to slowly drop. The blood test is usually measured as a trough level. It is timed so that the collection is 12 hours post dose and/or prior to the next dose at the drugs 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?Have the sample collected 12 hours after the last dose and immediately prior to the next dose or as directed by your doctor.
How is it used?The tacrolimus test is ordered to measure the amount of drug in the blood to determine whether concentrations have reached therapeutic levels and are below toxic levels. It is important to monitor levels of tacrolimus for several reasons:
- There is not a good correlation, as with some other medications, between the dose of tacrolimus given and level of drug in the blood.
- Absorption and metabolism of oral doses of tacrolimus can vary greatly between patients and even in the same patient depending on the time of the dose and what, if any, food has been eaten.
- Tacrolimus can cause kidney damage, especially in high doses. Measuring levels in patients who have had a kidney transplant may help to distinguish between kidney rejection (levels are low) and kidney damage due to tacrolimus toxicity (levels are high).
Tacrolimus monitoring helps ensure that each individual is receiving the right amount of drug needed to treat his or her particular case.
When is it ordered?Tacrolimus 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, then the frequency of tacrolimus testing may be decreased. The test is performed when dosages are changed and whenever patients have symptoms that suggest side effects, toxicity, or organ rejection. Monitoring at intervals is necessary as long as the patient is taking tacrolimus.
What does the test result mean?Concentrations that are higher than the established therapeutic range may cause symptoms associated with toxicity. Concentrations that are 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 higher tacrolimus levels.
Some signs and symptoms of tacrolimus toxicity include:
- Kidney damage
- High Blood Pressure
- Nausea and vomiting
- Electrolyte disturbances, such as hyperkalemia
- Tinnitusintermittent and/or persistent ringing or roaring in the ears
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 doctor will usually have his patients samples sent consistently to the same laboratory and will be guided by that laboratorys therapeutic ranges.
Is there anything else I should know?A variety of drugs, such as calcium channel blockers, antifungal drugs, macrolide antibiotics (such as erythromycin), protease inhibitors, chloramphenicol, and another immunosuppressant such as cyclosporine can increase tacrolimus concentrations. Drugs such as anticonvulsants (carbamazepine, phenytoin), antimicrobials (rifampin), and the immunosuppressant sirolimus can decrease tacrolimus concentrations.
Grapefruit juice and herbal supplements such as St. Johns Wort should be avoided while taking tacrolimus.
Tacrolimus may be given for a period of time to patients who have had bone marrow transplantation. 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. In some countries, tacrolimus is also licensed for use for myasthenia gravis.
Patients should not alter their dose or the time that they take their dose without consulting with their doctor. 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 doctor about associated risks.
Por quanto tempo vou precisar tomar tacrolimus?
Quem solicita os testes de tacrolimus?
Devo informar meu médico que estou tomando tacrolimus?
Sim, esta é uma informação importante, principalmente por causa das interações medicamentosas e parcialmente por causa da ação imunossupressora do tacrolimus. Ele pode afetar a capacidade de cicatrização, a eficácia de vacinas e pode aumentar o risco de desenvolver certos tipos de cânceres (como o câncer de pele).