Other names: TAC; FK506

Tacrolimus is a drug, which is given to patients who have had organ transplant surgery. Tacrolimus is an immunosuppressive drug, which helps to prevent the body from rejecting the donor organ; it temporarily suppresses the body’s immune system to prevent the body attacking the donor organ. Tacrolimus suppresses the immune system by reducing the activity of white blood cells known as T-lymphocytes.

The levels of tacrolimus in the blood must be kept within a small target range; if levels are too low or too high, this can lead to problems.

Why is the test used?

The test is used to measure the concentration of tacrolimus in the blood; this helps doctors to assess whether the dose is right and prevents the patient from having too little or too much tacrolimus in their body. If levels are too low, this may affect the efficacy of the drug and the body may start to attack the donor organ; if levels are too high, this may lead to symptoms associated with toxicity. It is important to measure the levels of the drug because people have different responses to the drug and the correlation between the dose of the drug and the concentration of the drug in the blood are not straightforward.

The test is carried out repeatedly when a patient starts tacrolimus therapy; this helps doctors to determine a suitable dosage. The test is always carried out if the dosage is changed and the test will be repeated if a patient shows symptoms of toxicity; symptoms include:

  • Seizures
  • Damage to the kidneys
  • Tremors
  • Headaches
  • Nausea
  • Vomiting
  • High blood pressure
  • Changes in the levels of electrolytes
  • Tinnitus (ringing in the ears)

How is the test done?

The test is done by taking a sample of the patient’s blood; the sample is usually taken from a vein on the inside of the elbow. The blood is drawn out using a needle and syringe; once the sample has been collected it will be bottled, labelled and sent off to the laboratory for testing.

The sample should be taken around 12 hours after the last dose or immediately before the next dose; the doctor will explain the timing of the test.

What do the test results mean?

If levels are too high, this may cause the patient to develop symptoms of toxicity; if levels are too low, this may cause the body to reject the donor organ.

Many types of medication may affect the concentrations of tacrolimus; drugs including antifungal drugs, cyclosporine, macrolide antibiotics and protease inhibitors can cause levels of tacrolimus to increase. Patients who are taking tacrolimus should not drink grapefruit juice or St John’s Wort.

Specific Blood Tests

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