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Detection of individuals with low thiopurine methyltransferase activity who are at risk for excessive myelosuppression or severe hematopoietic toxicity when taking azathioprine (Imuran) or 6-mercaptopurine (Purinethol)
Individuals who are either homozygous or heterozygous for thiopurine methyltransferase (TPMT) deficiency are at risk of developing life-threatening myelosuppression or severe hematopoietic toxicity when placed on standard doses of azathioprine (AZA, Imuran) or 6-mercaptopurine (6-MP, Purinethol).
Determining a patient's TPMT status prior to starting therapy with AZA or 6-MP is, therefore, important for purposes of calculating the optimal drug dosage.
There is partial overlap for the distribution of thiopurine methyltransferase activities observed between patients with normal and heterozygous genotypes. Results in the low-normal range can be due to biologic variation or assay variability.
Enzymatic End point/Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)