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A Guide to the TPMT Lab Test

written by: angiem1981•edited by: Diana Cooper•updated: 7/5/2011

The TPMT lab test, or the Thiopurine methyltransferase test, is a blood test used to check levels of the enzyme thiopurine before beginning certain therapies. This test can indicate whether or not thiopurine drugs will be an effective and safe treatment for the patient.

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    Why Might Your Physician Order This Test?

    If your physician has recommended thiopurine drugs for treatment of a disorder, the TPMT lab test will be needed to determine if this is the appropriate therapy. This test can determine whether or not there is sensitivity to this class of drugs and whether or not severe side effects from thiopurine will be a problem for you. Other than this, there are no other clinical implications for TPMT values in the body and scientists are still unsure of the exact role of this enzyme.

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    How is the Test Performed?

    Since thiopurine methyltransferase is measured in the red blood cells, it requires a sample of blood. You are not required to fast or take any special precautions prior to testing. Blood is drawn via a needle, typically in the arm. There are no risks associated with such tests, with the exception of bleeding, which is rare in most individuals. The procedure can be performed either in your physician’s office, hospital and sometimes local clinics.

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    How are the Results Read?

    The blood test measures the amount of thiopurine enzymes in the red blood cells. Although the exact values will differ between labs and testing methods, the result is measured in units per milliliter (U/ml) and are deemed either low or normal. It is rare to have high values of TPMT in the blood. When this occurs, it is believed that the effectiveness of thiopurine drugs is decreased. However, the implications of high values are currently still being studied.

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    What do the TPMT Test Results Mean?

    If you have normal levels of TPMT in the bloodstream, you may be a good candidate for thiopurine drug treatment and the normal amount may be given. Those that have no amounts of thiopurine enzyme activity, or very little may not be good candidates for thiopurine drugs and adverse effects are more likely. This means that your physician may not prescribe the particular drug, or only a very small dosage. However, if you have low to middle range activity, the drugs may prove to be more toxic and given in smaller amounts. As previously mentioned, high levels of activity are not common, but research indicates that those with high activity may have more tolerance for the drugs and they may not be as effective.

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    Special Considerations When Testing

    There are a few special considerations when it comes to the TPMT lab test, which can include factors that may alter results. This test should always be given prior to starting the thiopurine drug in question due to accuracy. In addition to this, those that have recently had a blood transfusion should alert the physician to this. Depending on how long ago the patient had the transfusion, they may have to wait to have the test performed.

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    Beaumont Laboratories Lab Test Directory. Beaumont Hospitals. 20, May 2008. Viewed 19, October 2010.

    TPMT. Lab Tests Online. Modified 18, June 2010. Viewed 19, October 2010.