Thiopurine s-methyltransferase

Other names: TPMT

Thiopurine drugs are used to suppress the immune system; they are used to treat patients who have had transplant surgery, as well as those with health conditions, including leukaemia, eczema and inflammatory bowel disease. Most people have the TPMT (thiopurine s-methyltransferase) enzyme, which helps to remove the drug from the body, but around 1 in 300 people do not have the enzyme; this means that they can develop very serious side-effects if they are given thiopurine drugs.

When is the test used?

The test is used before patients start therapy with thiopurine drugs; the test can check for the presence of the enzyme.

The test is usually ordered before a patient starts treatment with thiopurine drugs; the test may also be repeated if the patient is experiencing negative side-effects.

How is the test performed?

The test is performed by collecting and analysing a sample of the patient’s blood; a needle is inserted into the vein and the blood is drawn out and collected in a syringe. When the doctor (or nurse) has a sufficient sample, the blood will be bottled, labelled and sent away to the laboratory for analysis.

What do the test results mean?

If the test results shows no TPMT activity, this indicates that the patient may be at high risk of developing serious side-effects when given thiopurine drugs; if this is the case, an alternative medication will usually be prescribed. If the test result shows low TPMT activity, this will usually prompt the doctor to prescribe a lower dosage. If the result shows normal TPMT activity, this indicates that the patient will be able to take a normal dose of thiopurine.

Specific Blood Tests

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