This test is used to determine someone's potential responsiveness to clopidogrel, an antiplatelet medication, before a person starts taking the drug or during the initial treatment phase. Clopidogrel is a drug that is prescribed to prevent strokes and heart attacks in people who are at increased risk for these serious cardiovascular events. The drug works by helping to prevent harmful blood clots from forming by preventing platelets from clumping together (aggregating).
Clopidogrel genotype testing is used to detect variation in the CYP2C19gene that codes for one of the enzymes responsible for metabolizing clopidogrel into its active form. Variation in CYP2C19 is most often associated with reduced enzyme activity and decreased metabolism of clopidogrel, leading to a low level of active drug and potentially ineffective treatment for risk of blood clots. Testing for variants of the CYP2C19 gene is done to help tailor treatment. Those with certain genetic variations may require an alternative therapy that would treat their condition more effectively.
Clopidogrel-related genetic testing is not widely used at this time. Although studies have shown that CYP2C19 variation contributes to a person's responsiveness to clopidogrel, there is not yet consensus on the need for the testing. Part of the reason is that genetic variants of CYP2C19 explain only a small proportion of the treatment response variability among people.
Clopidogrel resistance testing is primarily ordered prior to a person taking clopidogrel for the first time or during the initial treatment phase, but may also be ordered when a person who is being treated with the drug has experienced either excessive blood clotting or bleeding.
Not everyone who is prescribed clopidogrel will have this test done. At present, there is no consensus on the usefulness of this test and it is not yet widely accepted.
Results of genetic testing require careful interpretation. Typically, the laboratory report will include an explanation from a doctor with expertise in this area. A number of factors are taken into account when determining whether clopidogrel is an appropriate treatment for an individual.
In general, if a person has one or two variant copies of the gene, then they may have an altered response to clopidogrel. The degree of responsiveness depends upon the variation(s) present and upon the person. An individual may be a normal, poor, intermediate, extensive, or ultra-rapid metabolizer of the drug.
Not every laboratory will test for every gene variant. A less common gene variant may be present that is not detected by the test. A person may also have decreased drug metabolism due to another factor. CYP2C19 gene variation only accounts for a portion of a person's response to clopidogrel.
To increase awareness of the influence of the CYP2C19 gene variations and the availability of testing, the U.S. Food and Drug Administration (FDA) has promoted changes to the Plavix® (clopidogrel) label to inform consumers that genetic factors may affect how well the drug works for them.
The enzyme CYP2C19, coded for by the CYP2C19gene, metabolizes a wide variety of drugs in addition to clopidogrel. Thus, if a person is taking clopidogrel plus one or more additional medications metabolized by CYP2C19, then that person may have altered metabolism of clopidogrel and the other drugs. One of the prominent examples of drugs that are metabolized by CYP2C19 is omeprazole that is taken to control stomach acid production. Others include diazepam, anti-seizure medications, and anti-depressants.
Testing to predict how someone will respond to a drug is a form of personalized medicine known as pharmacogenomics. It is a relatively new field of study developed in an effort to treat individuals' conditions effectively, yet avoid side effects.
This article was last reviewed on December 7, 2011. | This article was last modified on January 7, 2014.
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