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 drawn from a vein in your arm
Test Preparation Needed?
None
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. There are also 6-mercaptopurine and thioguanine.
One person in every 300 lacks thiopurine s-methyltransferase (TPMT) which is an enzyme that 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.
Some persons have reduced activity of TPMT and may need lower doses of the drugs. Others have enhanced activity of TPMT and the standard dose of the thiopurine drug may be less effective.
How is the sample collected for testing?
A blood sample is taken by a needle from a vein.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
The Test
How is it used?
TPMT is either measured in patients who are about to start treatment with a thiopurine drug such as azathioprine or if the doctor suspects that the patient is experiencing side effects due to TPMT deficiency The test identifies individuals at risk of developing severe side effects such as lowering of blood cell counts.
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 puts the patient at risk of less serious side effects such as hair loss, stomach pain, diarrhoea and inflammation of their pancreas. In this case the doctor may reduce the dose of thiopurine drug given.
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.
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.
