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Thiopurine S-Methyltransferase Blood Test

Last updated July 26, 2019

Approved by: Maulik P. Purohit MD, MPH

The Thiopurine S-Methyltransferase Blood Test is a blood test to assess the activity or production potential of TPMT in an individual.

What are the other Names for this Test? (Equivalent Terms)

  • TPMT (Thiopurine S-Methyltransferase Blood Test) Test
  • TPMT Genotype Blood Test
  • TPMT Phenotype Blood Test 

What is Thiopurine S-Methyltransferase Blood Test? (Background Information)

  • Thiopurine S-methyltransferase (TPMT) is an enzyme that breaks down a class of immunosuppressant drugs, called thiopurines. Thiopurine drugs include azathioprine and thioguanine. They function through various mechanisms to suppress an overactive or misdirected immune response
  • The immune system is responsible for defending the body against foreign invaders. These include bacteria, viruses, and other organisms. Any problem with the regulation of the immune system may lead to excessive responses that could damage the body’s own tissues. It may also cause the immune system to target the body’s own cells, as occurs with autoimmune diseases, such as Crohn’s disease and rheumatoid arthritis
  • Immunosuppressant drugs, such as thiopurines, reduce the activity of a dysfunctional immune system to minimize damage to the body. Once administered, thiopurines are targeted by TPMT for degradation, in an effort to return to homeostasis. Thus, the levels of TPMT produced by an individual, affect their sensitivity to thiopurine drugs
  • The Thiopurine S-Methyltransferase Blood Test is a blood test to assess the activity or production potential of TPMT in an individual. It is done to indirectly determine thiopurine drug sensitivity
  • TPMT enzyme activity is often measured from red blood cells. Also, because this is a test of an observable characteristic of the body, it is called a phenotype test
  • There are different versions of TMPT genes that cause production of TPMT enzymes that have different activity levels. TPMT production potential is estimated from the particular version of the TPMT gene that an individual possesses. The TPMT gene gives instructions for the TPMT enzyme, but different versions produce enzymes with different activities. Hence, this test is a genetic or genotype test 

What are the Clinical Indications for performing the Thiopurine S-Methyltransferase Blood Test?

The clinical indications for performing a Thiopurine S-Methyltransferase Blood Test include assessing the sensitivity of an individual to thiopurine drugs, prior to starting therapy. 

How is the Specimen Collected for Thiopurine S-Methyltransferase Blood Test?

Following is the specimen collection process for Thiopurine S-Methyltransferase Blood Test:

Sample required: Blood

Process: Insertion of a needle into an arm vein.

Preparation required: No special preparation is needed prior to the test. 

What is the Significance of the Thiopurine S-Methyltransferase Blood Test Result?

The significance of the Thiopurine S-Methyltransferase Blood Test is explained:

  • Normal TPMT enzyme activity or TPMT gene variants indicates that a standard dose is required to prevent toxicity and cause a therapeutic effect
  • Decreased TPMT enzyme activity or a nonstandard TPMT gene variant indicates that a smaller dose is required to prevent toxicity and cause a therapeutic effect 

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario. 

Additional and Relevant Useful Information:

  • Certain factors may interfere with the results of the Thiopurine S-Methyltransferase Blood Test. These include a recent blood transfusion and other red blood cell therapies 

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

References and Information Sources used for the Article:

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: July 17, 2014
Last updated: July 26, 2019