TPMT
Also known as: Thiopurine s-methyltransferase
The Test
How is it used?
TPMT is measured in patients who are about to start treatment with thiopurine drugs such as azathioprine. TPMT activity varies in the population and this means that different people require different doses of thiopurine drugs to get the desired therapeutic effects. Guidance in the UK recommends that patients commencing thiopurine drugs have their TPMT status checked before treatment begins. The test identifies individuals at risk of developing severe side effects such as lowering of blood cell counts and a lowered immune response.
When is it requested?
A doctor may requests a blood TPMT test before starting a patient on thiopurine drug treatment or if they suspect that existing side effects may be due to a deficiency of this enzyme.
What does the test result mean?
If a patient has no detectable TPMT activity then they are at risk of developing severe side effects to thiopurine drugs. Usually the doctor will find an alternative drug treatment. A low blood TPMT activity is still compatible with thiopurine drug treatment but a lower dose is indicated.
If a patient has normal blood TPMT activity then the doctor can treat the patient with a standard dose of a thiopurine drug.
If a patient has normal blood TPMT activity then the doctor can treat the patient with a standard dose of a thiopurine drug.
Is there anything else I should know?
The TPMT enzyme activity is measured in red blood cells so if you have recently received a transfusion of blood the results of this test may be inaccurate. The underlying basis to the variation is genetic differences in the population and when patients with no TPMT are identified their DNA is usually studied as well to confirm this status.
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