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TAC [Often referred to by brand name (see MedlinePlus Drug Information)]
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Este artigo foi modificado pela última vez em 10 de Julho de 2017.
At a Glance
Why Get Tested?
To determine the concentration of tacrolimus in the blood in order to establish a dosing regimen, maintain therapeutic levels, and detect toxic levels
When To Get Tested?
As soon as tacrolimus therapy begins, frequently at first then at regular intervals to monitor concentrations over time
Sample Required?
A blood sample drawn from a vein in your arm
Test Preparation Needed?
Have the sample collected 12 hours after the last dose and immediately prior to the next dose or as directed by your doctor.
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 recognize 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 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.
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Common Questions
  • 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
  • Tremors
  • Headache
  • High Blood Pressure
  • Nausea and vomiting
  • Electrolyte disturbances, such as hyperkalemia
  • Tinnitus—intermittent and/or persistent ringing or roaring in the ears
  • Seizures
  • 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.