Thiopurine methyltransferase (TPMT) deficiency is a condition in which patients treated with standard doses of azathioprine (AZA, Imuran), 6-mercaptopurine (6-MP, Purinethol), or 6-thioguanine (6-TG, Thioguanine Tabloid) may develop life-threatening myelosuppression or severe hematopoietic toxicity. Detection of individuals with low thiopurine methyltransferase (TPMT) activity who are at risk for excessive myelosuppression or severe hematopoietic toxicity when taking thiopurine drugs. Detection of individuals with hyperactive TPMT activity who have therapeutic resistance to thiopurine drugs and may develop hepatotoxicity if treated with these drugs. Ref. value: 3.00-6.66 nmol/mL/hour 6-Methylmercaptopurine (normal) 5.04-9.57 nmol/mL/hour 6-Methylmercaptopurine riboside (normal) 2.70-5.84 nmol/mL/hour 6-Methylthioguanine riboside (normal)