TPMT, Thiopurine S-Methyltransferase

This test is used to detect patients who are at risk of developing severe side effects if treated with the class of thiopurine drugs that includes azathioprine, mercaptopurine, and thioguanine. TPMT is measured in patients who are about to start treatment with a thiopurine drug, such as azathioprine, or if the doctor suspects that the patient is experiencing side effects due to TPMT deficiency. The test identifies individuals at risk of developing severe side effects, such as lowering of blood cell counts.

Thiopurine drugs suppress the immune system and are used to treat patients after organ transplants and in the treatment of a wide range of diseases, such as leukemia (especially pediatric acute lymphoblastic leukemia), inflammatory bowel disease, and skin disorders like eczema. The most commonly used thiopurine drug is called azathioprine.


A blood sample is obtained by inserting a needle into a vein in the arm.


Normal values are normal TPMT enzyme activity level. If you have a normal blood TPMT activity, your caregiver can treat you with a standard dose of a thiopurine drug.

Ranges for normal findings may vary among different laboratories and hospitals. You should always check with your doctor after having lab work or other tests done to discuss the meaning of your test results and whether your values are considered within normal limits.


Your caregiver will go over the test results with you and discuss the importance and meaning of your results, as well as treatment options and the need for additional tests if necessary.

If a patient has no detectable TPMT activity, they are at risk of developing severe side effects to thiopurine drugs. Usually, the caregiver will find an alternative drug treatment.

A low blood TPMT activity also puts the patient at risk of less serious side effects, such as hair loss, stomach pain, diarrhea, and inflammation of their pancreas. In this case, the caregiver may reduce the dose of thiopurine drug given.

The TPMT enzyme activity is measured in red blood cells, so if you have recently received a transfusion of blood, the results of this test may be inaccurate.


It is your responsibility to obtain your test results. Ask the lab or department performing the test when and how you will get your results.