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Thiopurine methyl transferase (TPMT)

Category Biochemistry
Test background

Thiopurine drugs are used to treat inflammatory and autoimmune disease, leukaemia and to prevent rejection post-organ transplantation. 6-mercaptopurine (6-MP), the major metabolite of azathioprine, is predominantly eliminated via the enzyme TPMT. TPMT activity has a wide inter-individual variation due to genetic polymorphism. In a Caucasian population, 89 % have normal activity and 11 % have intermediate levels. Approx 0.3 % have undetectable levels of TPMT and these patients are generally not treated with azathioprine, due to increased risk of severe side effects, eg. myelosuppression. Those patients with low activity usually receive a lower dose.

Clinical Indications

Prior to commencing azothioprine therapy.

Reference range

See report

Sample & container required EDTA (lavender top)
Sample volume 2 mL
Turnaround time Up to 3 weeks

Not currently performed in house; this is a referred test. Blood transfusion in last 3 months may mask complete TPMT deficiency. Please provide details at time of requesting so that genotyping can be performed instead. All patients should be advised prior to blood samples being taken that DNA confirmation may be performed.