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Why are blood cell counts monitored when a transplant recipient takes azathioprine?

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Kidney transplant — transplant British surgeon Andrew Ready flushing a donated kidney during a transplant at Queen Elizabeth Hospital, Birmingham, England, 2006.

Blood cell counts are monitored in transplant recipients taking azathioprine primarily to prevent damage to the blood-cell-forming tissues in the bone marrow.1

Because azathioprine works by inhibiting enzymatic pathways required for DNA synthesis, it can inadvertently suppress bone marrow function if the dosage is too high. Frequent monitoring of white blood cell and platelet counts is essential to ensure the dosage remains safe, as excessive suppression can lead to serious complications, including leukopenia (an abnormally low number of white blood cells), anemia, and hemorrhage. By tracking these levels, clinicians can adjust the medication to maintain the delicate balance required to prevent organ rejection while minimizing the risk of infection and bleeding.21