Tacrolimus (FK506) Trough Level Test After Organ Transplant

This test measures the amount of tacrolimus (FK506), an immunosuppressive medicine, in your blood at its lowest point in the dosing cycle, known as the trough. Tacrolimus helps keep your immune system from attacking your transplanted organ.

The ABO and Rh blood type test identifies your blood group (A, B, AB, or O) and your Rh status (positive or negative). After a hematopoietic stem cell transplant, this test is performed repeatedly because your blood type may change to match that of your stem cell donor. The transplanted stem cells gradually produce red blood cells with the donor's blood type antigens.

After a transplant, your immune system may see the new organ as foreign and try to reject it. Taking the right amount of tacrolimus lowers this risk, but too much can damage your kidneys or raise the chance of infection. Regular monitoring keeps your dose balanced and safe.

After a stem cell transplant, your body is rebuilding its blood cell production system. The new stem cells from your donor will eventually produce red blood cells with different antigens than what you were born with. This means your blood type can change over time, typically over several months. Knowing your current blood type is critical for receiving safe blood transfusions, as receiving incompatible blood can cause serious reactions.

A health professional draws a small sample of blood from a vein in your arm, usually just before your next scheduled dose of tacrolimus, when the level in your blood is lowest. The sample is sent to a laboratory, and results are typically ready within one to two days.

A healthcare provider will draw a small sample of blood from a vein in your arm. The sample is then tested in a laboratory to determine your ABO blood group and Rh factor. This test may be repeated multiple times after your transplant to track any changes in your blood type as the donor stem cells engraft and begin producing new blood cells.

If your result falls within the target range set by your transplant team, your current dose is likely working well. A level below the range may raise the risk of organ rejection, while a level above it can cause side effects such as kidney problems or infections. Your doctor will adjust your dose if needed.

Your test results will show your current ABO blood type (A, B, AB, or O) and whether you are Rh-positive or Rh-negative. After transplant, your healthcare team will compare your current results to your pre-transplant type and monitor for changes. If your type begins to match your donor's type, this is a normal sign that the transplant is working and the donor cells are producing new blood cells.