CPT® Code 84433

Thiopurine S-methyltransferase (TPMT) is a critical enzyme involved in the metabolism of thiopurine drugs, which are cytotoxic agents commonly used in the treatment of conditions such as leukemia and inflammatory bowel disease. These drugs function by suppressing the immune system, making the assessment of TPMT activity essential prior to initiating treatment. The CPT® code 84433 specifically refers to the laboratory test that measures the activity of TPMT in red blood cells. This blood test is particularly important for identifying patients who may have low or intermediate levels of TPMT activity, as these levels can significantly influence the safety and efficacy of thiopurine therapy. When a patient has low TPMT activity, there is a heightened risk of severe bone marrow toxicity, which can lead to myelosuppression—a potentially life-threatening condition characterized by a decrease in the production of blood cells. Consequently, if a patient is found to have low TPMT activity, it is recommended that thiopurine treatment be avoided altogether. On the other hand, patients with intermediate TPMT activity may still be candidates for thiopurine therapy, but they will require careful dose adjustments and close monitoring to mitigate the risk of adverse effects. The testing for TPMT enzymatic activity is performed using advanced techniques such as quantitative liquid chromatography-tandem mass spectrometry, ensuring accurate and reliable results that guide clinical decision-making in the management of patients requiring thiopurine medications.