TPMT
At a Glance
Why Get Tested?
The detection of patients who are at risk of developing severe side effects if treated with the thiopurine drugs, azathioprine, mercaptopurine and thioguanine
When to Get Tested?
Prior to thiopurine drug treatment
Sample Required?
A single blood sample taken from a vein in your arm
Test Preparation Needed?
There are no special preparations needed to collect the blood.
The Test Sample
What is being tested?
Thiopurine drugs suppress the immune system and are used to treat patients after organ transplants and in the treatment of a wide range of diseases such as leukaemia, inflammatory bowel disease and skin disorders such as eczema. The most commonly used thiopurine drug is called azathioprine.
1 person in every 300 lacks thiopurine s-methyltransferase (TPMT) which is an enzyme which helps remove thiopurine drugs such as azathioprine from the body. Individuals with no TPMT enzyme can become severely ill if treated with normal doses of thiopurine drugs. These patients develop bone marrow suppression with a reduction in blood cells, which causes anaemia, infection and abnormal bleeding. Such side effects can be avoided if TPMT is measured before starting treatment.
How is the sample collected for testing?
A blood sample is taken by a needle from a vein.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
There are no special preparations needed to collect the blood.
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.
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.
Common Questions
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Article Sources
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
