Thiopurine Methyltransferase Testing (TPMT)

What is Thiopurine Methyltransferase Testing (TPMT)

Thiopurine Methyltransferase testing helps determine if you have enough TPMT enzymes to take thiopurine medicines or what doses you should be taking. Thiopurine methyltransferase (TPMT) is an enzyme that you need to break down medicines called thiopurines.

If you have enough enzymes, you should have fewer side effects from the medicines. If you do not have enough enzymes, these medicines can build up in your system and damage the cells inside your bones (bone marrow) that make blood cells.

Thiopurine medicines include azathioprine, mercaptopurine, and thioguanine. You may need one of these medicines if you have:

  • Certain types of blood cancers. This includes acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML).
  • A kidney transplant.
  • Refractory rheumatoid arthritis.

What is being tested?

TPMT testing may involve two different tests.

  • TPMT phenotyping measures TPMT enzyme activity in your blood.
  • TPMT genotyping checks for gene abnormalities that lead to TPMT deficiency.

What kind of sample is taken?

A blood sample is required for the TPMT phenotyping test. The sample is usually collected by inserting a needle into a blood vessel.

TPMT genotyping can be done on a blood sample or by taking a swab of tissue from your mouth.

How do I prepare for this test?

  • TPMT phenotyping should be done before starting any thiopurine medicine because taking the medicine before the test will alter the results.
  • No preparation is needed for TPMT genotyping.

Tell a health care provider about:

  • All medicines you are taking, including vitamins, herbs, eye drops, creams, and over-the-counter medicines.
  • Any medical conditions you have.

How are the results reported?

The results of TPMT phenotyping will be a value that indicates the amount of TPMT enzyme activity in your blood. TPMT phenotyping may be measured in units per milliliter (units/mL). Your health care provider will compare your results to normal ranges that were established after testing a large group of people (reference ranges). Reference ranges may vary among labs and hospitals. For this test, a common reference range is 25–65 units/mL.

The results of TPMT genotyping will be reported as homozygous normal, homozygous, or heterozygous with variants. Homozygous normal is considered a normal result.

What do the results mean?

  • TPMT phenotyping. Results that are within the reference range are considered normal. Abnormally high levels may indicate that a higher dose of thiopurine medicine is needed. Abnormally low levels may indicate that a lower dose of thiopurine medicine is needed. Low levels may also indicate the need to use a medicine other than thiopurine medicine.
  • TPMT genotyping. A result of homozygous normal means that no gene abnormalities were found and that you probably have a normal amount of TPMT enzyme. Abnormal TPMT genotyping results may indicate that you have an increased risk for damage to your bone marrow with the use of thiopurine medicine.

Talk with your health care provider about what your results mean.

Questions to ask your health care provider

Ask your health care provider, or the department that is doing the test:

  • When will my results be ready?
  • How will I get my results?
  • What are my treatment options?
  • What other tests do I need?
  • What are my next steps?

Summary

  • Thiopurine methyltransferase (TPMT) is an enzyme that you need to break down medicines called thiopurines. TPMT testing helps determine if you have enough TPMT enzymes to take these medicines or what doses you should be taking.
  • If you have enough enzymes, you should have fewer side effects from thiopurine medicines. If you do not have enough enzymes, these medicines can build up in your system and damage the cells inside your bones (bone marrow) that make blood cells.
  • Talk with your health care provider about what your results mean.
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