The TPMT blood test is used to test the activity of the thiopurine methyltransferase enzyme. Sometimes the test is ordered to measure the genetic structures that are in place supporting the TPMT activity instead. There are two primary reasons for having this test ordered.
1.To determine if starting treatment with a thiopurine drug is suitable.
2. To discover if there would be a high likelihood of severe side effects should a thiopurine drug be administered for therapy.
Thiopurine drugs are used most often for autoimmune disorders, such as rheumatoid arthritis. It is also commonly used for organ transplant recipients and when a diagnosis of Crohn’s disease has been confirmed. The purine antimetabolites might also be used should acute lymphoblastic leukemia need to be treated.
The TPMT Blood Test Has 2 Forms
A doctor may ordered a phenotype blood test or a genotype blood test to receive the needed TPMT results. Occasionally both tests may be ordered. The phenotype blood test is much more commonly ordered than the genotype blood test. Each test has results that mean specific things.
The Phenotype TPMT Blood Test
This blood test is usually ordered before someone is about to begin a thiopurine drug therapy. If the results show that there is no detectable enzyme activity or that the results are below normal, then there is a high risk of developing a severe side effect from the proposed drug therapy. Low results almost always result in an alternative treatment being proposed.
When the results are close to normal, but are still classified as low, then there is still a risk of havingg toxicity occur due to the drug treatment. This blood test result will typically bring about a lower dose of the thiopurine drug to treat the underlying disease or disorder.
Normal results mean that a doctor can treat a patient with a standard dose of a thiopurine drug on a regular basis. There are no counter-indications of a TPMT blood test being above average, although it is believed that having high levels of enzyme activity could prevent a thiopurine drug being as effective as it could be.
The Genotype TPMT Blood Test
This blood test is ordered to determine if there are variations within the TPMT gene that could contribute to low levels of enzyme activity. Instead of measuring the actual activity levels, the test looks to see if there are one of two known “rogue” copies of the TPMT gene or if one of the TPMT genes is associated with a negative gene variation.
When there are two genes with variations, a standard dose of a thiopurine drug is typically used as enzyme activities will be normal. Having a heterozygous gene variation is associated with severe side effects to a thiopurine treatment in up to 60% of people who receive it, so a positive result here would lead a doctor to a different treatment for the underlying disease or disorder.
Some people have a homozygous result from this blood test, which means they have two variant genes that produce almost no enzyme activity. With this result, there is a 100% chance of developing toxicity within the bone marrow should a thiopurine drug be administered.
Rare variants are not necessarily detected by this test. If reduce enzyme activity is suspected, then bot tests may be ordered to confirm the diagnosis.
What You Need To Know About the TPMT Blood Test
TPMT enzyme activity can be artificially reduced for a wide variety of reasons. Your doctor will likely request that you abstain from NSAID pain relievers, certain diuretics, and benzoic acid inhibitors for at least 48 hours before the TPMT blood test. People who have had a recent blood transfusion also tend to test artificially low for enzyme activity.
The TPMT blood test is usually given in conjunction with a complete blood count. The overall goal is to track certain blood cells to measure the effectiveness of a treatment. People who are receiving thiopurine drugs and have a low white blood cell count may have the TPMT blood test ordered a second time to confirm first-time results. It is not used for tracking purposes, but is instead used in conjunction with complete blood count results.
The results of the TPMT blood test will help to determine what method of treatment is best for an individual’s current health. Be sure to discuss any results received with a medical professional and consider receiving a second opinion about any underlying condition before proceeding with a treatment plan that is formed, in part, by this blood test.