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Thiopurine methyltransferase (TPMT) test is used to measure the Thiopurine methyltransferase (TPMT) enzyme activity. This test is ordered by doctors before they start to treat a patient with thiopurine drugs.  This test helps to determine patients who are at high risk of suffering from side effects of thiopurine drugs.

Patients with low or medium levels of TPMT enzyme activity are at high risk of suffering from side effects of thiopurine drugs as the drug levels will increase to high amounts in the blood as it is not broken down quickly by the body. Such patients are either prescribed alternative medicines or are given reduced dosages of thiopurine drugs.