At a Glance

Why Get Tested?

To detect a thiopurine methyltransferase (TPMT) deficiency and determine your risk of developing severe side effects if treated with the class of immune-suppressing thiopurine drugs that includes azathioprine, mercaptopurine, and thioguanine

When To Get Tested?

Typically, prior to thiopurine drug treatment; this is a specialized test and is not routinely ordered.

Sample Required?

A blood sample drawn from a vein in your arm or a swab from inside your cheek (i.e., buccal swab)

Test Preparation Needed?

For measuring TPMT enzyme activity (TPMT phenotyping), the test must be performed prior to taking a thiopurine drug since it may affect results; for the genetic test (TPMT genotyping), no specific test preparation is needed.

What is being tested?

Thiopurine methyltransferase (TPMT) is an enzyme that breaks down (metabolizes) a class of drugs called thiopurines. These drugs are used to suppress the immune system and are prescribed to treat various immune-related conditions or blood disorders (e.g., leukemia). The activity level of the TPMT enzyme, or the genetics underlying the enzyme’s activity, is tested before thiopurine drug therapy to make sure that individuals treated with the drugs can metabolize them.

Examples of thiopurines include azathioprine, mercaptopurine, and thioguanine. These medications are used to treat diseases such as acute lymphoblastic leukemia, inflammatory bowel disease, and autoimmune disorders. They may also be prescribed for organ transplant recipients to help delay or prevent organ rejection. If someone’s TPMT activity is too low, the person may not effectively metabolize thiopurines, which can lead to severe side effects.

About one person in every 300 is severely deficient in TPMT, and about 10% of the population in the U.S. has lower than normal levels of TPMT. Individuals in both categories are at an increased risk for thiopurine drug toxicity, which can affect the activity of the bone marrow (myelosuppression) and lead to very reduced levels of blood cells, such as red blood cells, white blood cells and platelets (hematopoetic toxicity). This can lead to complications such as anemia, serious infections, and/or excessive bleeding.

These side effects can cause an individual to become severely ill and may even be life-threatening. These side effects can be avoided if TPMT testing is done before starting thiopurine treatment.

There are two ways to determine whether an individual is at risk of side effects from thiopurine therapy:

  • TPMT activity test (phenotype)—this method tests the activity level of the enzyme thiopurine S-methyltransferase (TPMT) in a person’s red blood cells. Depending on the enzyme activity level, a person may be prescribed a standard dose of the thiopurine drug, a reduced dose of the thiopurine drug, or a different drug other than a thiopurine.
  • TPMT genetic test (genotype)—an alternative test to TPMT enzyme activity level is a genetic test that can identify genetic variations in the TPMT gene. This genetic test identifies individual genetic differences associated with risk for thiopurine toxicity. Each person has two copies of the TPMT gene. Most people have two copies of “wild type” TPMT that produce sufficient TPMT enzyme. Approximately 10% of people have one copy of the wild-type gene and one copy of a gene variation associated with decreased TPMT (heterozygous) and intermediate enzyme activity. Approximately one in 300 individuals have two copies of TPMT with variations resulting in little or no enzyme activity (homozygous). While numerous variations can occur in TPMT, there are five variations in particular that have been proven to be associated with TPMT deficiencies. Most genetic tests look for these five variations, although depending on the method used, more variations can be detected.This genetic test provides information about a person’s likely response to thiopurines, but it will not measure how much TPMT enzyme is actually being made by the body. There can be significant person-to-person and ethnic variability in TPMT production, even in people with the same gene variations.

A third, different kind of test may be used after thiopurine treatment begins. A test that measures thiopurine breakdown products (thiopurine metabolites) may be used to monitor therapy and adjust doses.

How is the sample collected for testing?

For both genetic and enzyme activity testing, a blood sample is taken by needle from a vein in the arm. Alternatively, for the genetic test alone, a swab of cells from the inside cheek (buccal swab) may be collected.

Is any test preparation needed to ensure the quality of the sample?

For the method that measures TPMT enzyme activity, taking a thiopurine drug could lead to falsely low results, so the test should be performed prior to starting therapy. For the genetic test (TPMT genotyping), no specific test preparation is needed.

Common Questions

How is it used?

The tests for thiopurine methyltransferase (TPMT) enzyme activity or its underlying genetics are measured in people who are about to start treatment with a thiopurine drug. One or the other of these tests is used to identify individuals at risk of developing severe side effects from thiopurine therapy, such as suppression of the bone marrow that leads to reduced numbers of red blood cells, white blood cells and platelets.

Thiopurines such as azathioprine, mercaptopurine, and thioguanine are drugs that are prescribed for diseases such as acute lymphoblastic leukemia (ALL), inflammatory bowel disease, and autoimmune disorders. They may also be prescribed for organ transplant recipients to help prevent organ rejection.

When is it ordered?

A healthcare practitioner will typically order a TPMT enzyme activity test or genetic test before starting a person on thiopurine drug treatment. Occasionally, a TPMT genotype test may be ordered when a person treated with a thiopurine drug experiences side effects, such as a decreased WBC count.

What does the test result mean?

Phenotype test for TPMT

  • If someone has little to no detectable TPMT activity, that person is at risk of developing severe side effects to thiopurine drugs. Usually the health care practitioner will find an alternative drug treatment. Sometimes the healthcare practitioner may prescribe a very small dose of the thiopurine.
  • Low to intermediate TPMT activity also puts individuals at increased risk for toxicity. In this case, the healthcare practitioner may reduce the dose of thiopurine drug given.
  • If someone has normal TPMT activity, the health care practitioner can treat the person with a standard dose of a thiopurine drug.

Genotype test for TPMT

  • A genetic test to detect genetic variations in the TPMT gene will help determine TPMT activity and risk for side effects from low TPMT activity.
  • Individuals with two “wild type” copies of the TPMT gene produce sufficient TPMT and have little risk of thiopurine toxicity. Most people fall into this category and can be treated with a standard dose.
  • People who have one normal gene and one gene variation associated with decreased TPMT (heterozygous) may produce an intermediate amount of TPMT. Approximately 30-60% of people who are heterozygous have severe side effects from standard doses of thiopurines. They will likely require reduced doses of the drug but may need to be given an alternative drug.
  • People with two copies of a variant TPMT gene (homozygous) and who produce little to no TPMT have 100% likelihood of developing severe bone marrow toxicity (myelosuppression) when treated with conventional doses of thiopurines. They will likely be given an alternative drug.
  • The genetic test usually detects the most common variants associated with TPMT deficiency. It is possible for a person to have a rare variant not detected by this test, who may subsequently experience serious side effects from treatment with a thiopurine drug.

Is there anything else I should know?

Though the TPMT test is used to predict risk of bone marrow toxicity, complete blood counts (CBCs) should also be done at regular intervals to detect bone marrow toxicity during treatment with thiopurine drugs. The CBC is a common blood test that measures the number of red blood cells, white blood cells and platelets. It is often used to monitor drug treatments that are known to affect the bone marrow.

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.

Besides your genetic makeup, there may be other reasons for increased risk of bone marrow toxicity (myelosuppression) from treatment with thiopurine drugs. Interactions between certain drugs can also inhibit TPMT enzyme activity. These drugs include naproxen, ibuprofen, ketoprofen, furosemide, sulfasalazine, mesalamine, olsalazine, mefenamic acid, thiazide diuretics, and benzoic acid inhibitors. TPMT inhibitors may contribute to falsely low results; people should not take these drugs for at least 48 hours prior to TPMT testing.

A few people may have high levels of TPMT activity. It is thought that this decreases the effectiveness of thiopurine therapy, and these people may be treated with an alternative drug.

My health practitioner ordered a test for thiopurine metabolites. What is it and how is it related to TPMT testing?

A health practitioner may order a blood test for thiopurine metabolites to monitor drug therapy. Measuring the metabolites is another way to ensure that toxic levels do not build up in the blood. Prior to administering the first dose, a healthcare practitioner may test a person’s TPMT enzyme activity or genotype to help determine risk of side effects as described in other sections of this article. The healthcare practitioner can adjust the prescribed dose according to those results. After therapy begins, the level of metabolites can be measured and monitored, with subsequent doses adjusted as necessary to avoid toxicity.

What are the pros and cons of the TPMT phenotype test and the genotype test?

The opinions of medical organizations vary on which TPMT test to recommend. You should consult with your healthcare practitioner about which one best fits your individual situation. For example:

  • The U.S. Food and Drug Administration (FDA) and prescribing information for azathioprine and mercaptopurine recommend either TPMT genotyping or phenotyping prior to thiopurine treatment.
  • The American College of Gastroenterology prefers TPMT phenotyping for people who are being treated with thiopurines for inflammatory bowel disease because the phenotype test measures the level of TPMT enzyme activity.


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