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Results
By Mayo Clinic staffIt usually takes several days to get the results of cytochrome P450 tests. Once they're back, you and your doctor will discuss the results and how they might affect your treatment options.
Cytochrome P450 tests are used to determine how well you process (metabolize) a drug. The results of cytochrome P450 tests divide people into four main types:
- Poor metabolizers. If you metabolize a certain drug more slowly than normal, the medication can build up in your system. This increases the likelihood that it will cause side effects.
- Normal metabolizers. If the test shows that you process certain antidepressants normally, you're more likely to benefit from treatment and to have fewer side effects than are people who don't metabolize those particular medications as well.
- Intermediate metabolizers. Because you have at least one gene involved in drug metabolism that doesn't function normally, you may not process some medications as well as normal metabolizers do.
- Ultrarapid metabolizers. In this case, medications leave your body too quickly — often before they have a chance to work properly.
Cytochrome P450 testing isn't useful for all antidepressants, but it can provide information about how you're likely to process a number of them. These include:
- Fluoxetine (Prozac)
- Duloxetine (Cymbalta)
- Fluvoxamine (Luvox)
- Paroxetine (Paxil)
- Venlafaxine (Effexor)
- Desvenlafaxine (Pristiq)
- Nortriptyline (Pamelor)
- Amitriptyline
- Clomipramine (Anafranil)
- Desipramine (Norpramin)
- Imipramine (Tofranil)
Test limitations
Although they have potential, genotyping tests do have limitations. Cytochrome P450 testing can't predict for certain which particular medication will work best for you — it can only provide clues. Cytochrome P450 tests only look at some of the genes involved in how your body uses certain drugs — so there are still factors involved that may affect how an antidepressant will affect you that aren't indicated by cytochrome P450 testing.
Because they're still being developed, it isn't entirely clear how useful cytochrome P450 tests are in choosing an antidepressant. An advisory group formed by the Centers for Disease Control and Prevention has concluded that there isn't enough evidence yet to support using cytochrome P450 testing for the most commonly prescribed antidepressants. But some doctors do use them and think they're helpful. Trying antidepressants based on a medical history and symptoms is still the standard method for identifying the best medication.
If you opt for testing, keep in mind that the cytochrome P450 tests aren't meant to be the only way to determine which antidepressants to try. They're just one tool that may help. Even if you have cytochrome P450 tests, you may still need to try different antidepressants and different doses to identify what works best for you.
- Mrazek DA. Psychiatric pharmacogenomic testing in clinical practice. Dialogues in Clinical Neuroscience. 2010;12:69.
- Li X, et al. Clinical utility of microarrays: Current status, existing challenges and future outlook. Current Genomics. 2008;9:466.
- Zandi PP, et al. The promise and reality of pharmacogenetics in psychiatry. Psychiatry Clinics of North America. 2010;33:181.
- Black JL (expert opinion). Mayo Clinic, Rochester, Minn. May 4, 2010.
- Evaluation of genomic applications in practice and prevention (EGAPP) working group. Recommendations from the EGAPP Working Group: Testing for cytochrome P450 polymorphisms in adults with nonpsychotic depression treated with selective serotonin reuptake inhibitors. Genetics in Medicine. 2007;9:819.
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