TPMT
Note: this site is for informational purposes only. To view test results or book a test, use the NHS app in England or contact your GP.
A TPMT (thiopurine methyltransferase) test measures the activity of the TPMT enzyme or detects genetic variations affecting it using a blood sample taken from a vein in the arm. It is used to assess how a person will respond to thiopurine drugs, helping to guide dosing and reduce the risk of serious side effects such as bone marrow suppression.
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.
Before taking a sample for TPMT activity, patients may be advised that DNA confirmation may be performed. The only known implication for the genetic variation in TPMT expression is intolerance to thiopurine drugs.
Common questions
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.
A doctor may request 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.
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 TPMT activity is still compatible with thiopurine drug treatment but a lower dose is indicated.
If a patient has normal TPMT activity then the doctor can treat the patient with a standard dose of a thiopurine drug.
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 activity are identified their DNA (for TPMT genotyping) is usually studied as well to confirm this status.
Globally the other genetic variation that can result azathioprine toxicity is in NUDT15 gene. This test has recently become available for people with acute lymphoblastic leukaemia about to receive azathioprine but hopefully will become more widely available in the future.
No one really knows the answer to this. The enzyme is found in many cells but the interest in TPMT is purely in looking at patients who are to receive thiopurine drugs as this enzyme breaks down these drugs to inactive metabolites. Around 1 in 300 people have no TPMT activity and a further 11% have low activity.