At a Glance
Why Get Tested?
To detect patients who are at risk of developing severe side effects if treated with the class of thiopurine drugs that includes azathioprine, mercaptopurine, and thioguanine
When to Get Tested?
Prior to thiopurine drug treatment
A blood sample drawn from a vein in your arm
Test Preparation Needed?
The Test Sample
What is being tested?
Thiopurines are a class of drugs that suppress the immune system. Examples 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 to patients who have had organ transplants to help delay or prevent organ rejection. This test detects the activity level of the enzyme thiopurine S-methyltransferase (TPMT) in a person’s red blood cells. The activity level of TPMT is associated with the ability to effectively metabolize thiopurines.
Thiopurines must be activated and then inactivated in the body. TPMT is one of the crucial components of deactivation. If there is not a sufficient amount of TPMT present, then toxic active drug compounds can accumulate in the blood. About one person in every 300 is severely deficient in TPMT, and about 10% of the population has lower than normal levels. Individuals with intermediate amounts of TMPT are at an increased risk for drug toxicity. Those with little to no TPMT enzyme can develop suppression of their bone marrow (myelosuppression) and become severely ill if treated with normal doses of thiopurine drugs. This suppression leads to a reduction in the production of red and white blood cells, which in turn can cause infection and abnormal bleeding. Such side effects can be avoided if TPMT is measured before starting treatment.
TPMT activity levels can be affected by variations (mutations) in the genes that code for the production of TPMT. There is a one-time genetic test that can be done on the DNA of an individual. Tests typically detect the most common gene mutations, but do not test for all of them and will not account for TPMT gene variations that have not yet been identified. This test can give a doctor information about her patient’s likely response to thiopurines, but it will not quantify how much TPMT is actually being made by the body. There can be significant person-to-person and ethnic variability in TPMT production, even when people have the same gene variations. When it is ordered, this genetic test is usually performed prior to thiopurine therapy. However, it is not affected by thiopurine therapy and can be performed at any time.
How is the sample collected for testing?
A blood sample is taken by a needle from a vein in your arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
Sources Used in Current Review
Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC. Pp. 479.
Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp. 1420.
(2008 July). Thiopurine Methyltransferase, Red Blood Cell. ARUP Laboratories Technical Bulletin [On-line information]. PDF available for download at http://www.aruplab.com/Testing-Information/resources/TechnicalBulletins/Thiopurine%20Methyltransferase,%20Red%20Blood%20Cell.pdf through http://www.aruplab.com. Accessed on 1/4/09.
Ansari, A.et. al (2008 December 17). Prospective Evaluation of the Pharmacogenetics of Azathioprine in the Treatment of Inflammatory Bowel Disease. [On-line information]. Available online at http://www.medscape.com/viewarticle/584257 through http://www.medscape.com. Accessed on 1/4/09.
Aberra, F. and Lichtenstein, G. (Review Article: Monitoring of Immunomodulators in Inflammatory Bowel Disease. Medscape from Aliment Pharmacol Ther. 2005; 21 (4): 307-319. [On-line information]. Available online at http://www.medscape.com/viewarticle/499922_1 through http://www.medscape.com. Accessed on 1/4/09.
Derijks, L. et. al. (2006 September 26). Thiopurines in Inflammatory Bowel Disease. [On-line information]. Available online at http://www.medscape.com/viewarticle/543562 through http://www.medscape.com. Accessed on 1/4/09.
Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER and Bruns DE, eds. 4th ed. St. Louis, Missouri: Elsevier Saunders; 2006, P. 1273.
Sources Used in Previous Reviews
Dr. Jonathon Berg. City Hospital, Birmingham, England.