What is being tested?
This test measures the amount of tacrolimus in the blood. Tacrolimus is an immunosuppressive drug that is given orally or intravenously to patients who have had a kidney, liver, heart, or other organ transplant. Normally, a person's immune system would recognise the new organ as foreign and begin to attack it. Tacrolimus limits this response and helps to prevent organ rejection by inhibiting the activation of T-lymphocytes.
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 drug's lowest concentration in the blood.
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:
When is it requested?
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 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:
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 patient's samples sent consistently to the same laboratory and will be guided by that laboratory's therapeutic ranges.
Monitoring at intervals is necessary as long as someone is taking tacrolimus.
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. John's 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.
Tacrolimus increases the risk of developing malignancies such as lymphoma and skin cancer, so sunlight should be avoided when taking this drug.
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.
Common questions
Transplant patients must take tacrolimus or a different immunosuppressant (or combination of drugs) throughout their life. Talk to your doctor about the best choice for your condition. This choice may change over time.
Tacrolimus will usually be monitored by the doctor and transplant team that performed your surgery or by a doctor who has specific knowledge of the condition or disease for which the drug is prescribed.
Yes, this is important information for them, partially because of drug interactions and partially because of the immunosuppressive action of tacrolimus. It can affect your ability to heal, to get vaccinations, and may increase the risk of developing certain cancers (such as skin cancers).
More Information
Pathology Tests Explained (PTEx) is a not-for profit group managed by a consortium of Australasian medical and scientific organisations.
With up-to-date, evidence-based information about pathology tests it is a leading trusted sources for consumers.
Information is prepared and reviewed by practising pathologists and scientists and is entirely free of any commercial influence.
