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Genetic testingBy Mayo Clinic staff
Original Article: http://www.mayoclinic.com/health/genetic-testing/MY00370
Genetic testing involves examining your DNA, the chemical database that carries instructions for your body's functions. Genetic testing can reveal changes or alterations in your genes that may cause illness or disease.
Although genetic testing can provide important information for diagnosing, treating and preventing illness, there are limitations. For example, if you're a healthy person, a positive result from genetic testing doesn't always mean you will develop a disease. On the other hand, in some situations, a negative result doesn't guarantee that you won't have a certain disorder.
Talking to your doctor or a genetic counselor about what you will do with the results is an important step in the process of genetic testing.
Why it's done
Several types of genetic testing are done for different reasons:
- Diagnostic testing. If you have symptoms of a disease that may be caused by genetic alterations, genetic testing can reveal if you have the suspected disorder. Examples of disorders for which genetic testing may be used to confirm a diagnosis include adult polycystic kidney disease, iron overload (hemochromatosis) and Charcot-Marie-Tooth disease.
- Presymptomatic and predictive testing. If you have a family history of a genetic condition, undergoing genetic testing before you have symptoms may show if you're at risk of developing that condition.
- Carrier testing. If you or your partner has a family history of a genetic disorder, such as sickle cell anemia or cystic fibrosis, you may choose to have genetic testing before you have children. This type of genetic testing may also be useful if you are in an ethnic group that has a high risk of a particular genetic disorder. Genetic testing can determine if you carry a copy of an altered gene that would put a child at risk of developing the disorder.
- Pharmacogenetics. If you have a particular health condition or disease, this type of genetic testing may help determine what medication and dosage will be most effective and beneficial for you.
- Prenatal testing. If you are pregnant, tests are available that can detect some types of abnormalities in your fetus's genes. Spina bifida and Down syndrome are two genetic disorders that are often screened for as part of prenatal genetic testing.
- Newborn screening. This is the most common type of genetic testing. In the United States, all states require that newborns be tested for certain gene abnormalities that cause specific conditions. This type of genetic testing is important because if results show there's a disorder such as congenital hypothyroidism or phenylketonuria (PKU), care and treatment can begin right away.
- Preimplantation testing. This technique may be used to lower the chances that you'll have a child with a particular genetic disorder. The test, also called preimplantation genetic diagnosis (PGD), requires that you conceive a child through in vitro fertilization. With in vitro fertilization, eggs are removed from a woman and sperm are collected from a man. The eggs are then fertilized with the sperm outside the body to create embryos. The embryos are then screened for genetic abnormalities. Embryos without abnormalities are implanted in the uterus in hopes of achieving pregnancy.
How you prepare
Before you undergo genetic testing, gather as much information as you can about your family's medical history. Then, talk with your doctor or a genetic counselor about your personal and family medical history. This will help you better understand your risk. Discuss any questions or concerns you have about genetic testing at that meeting. Also, talk about your options, depending on the results of the test.
If you are being tested for a genetic disorder that runs in families, you may want to consider discussing your decision to undergo genetic testing with your family. Having these conversations before testing can give you a sense of how your family might respond to your test results and how it will affect them.
Not all health insurance pays for genetic testing. So, before you have a genetic test, check with your insurance provider to see what will be covered. In the United States, the federal Genetic Information Nondiscrimination Act (GINA) helps prevent health insurers or employers from discriminating against you based on test results. Most states offer additional protection.
What you can expect
Your doctor, medical geneticist or nurse practitioner may select, order or administer a genetic test. A sample of your blood, skin, amniotic fluid or other tissue will be collected and sent to a lab for analysis.
- Blood sample. A member of your health care team obtains the sample by inserting a needle into a vein in your arm. For newborn screening tests, a blood sample is taken by pricking your baby's heel.
- Cheek swab. In some situations, a swab sample from the inside of your cheek is collected for genetic testing.
- Amniocentesis. This prenatal genetic test relies on a sample of the amniotic fluid. During amniocentesis, your doctor inserts a thin, hollow needle through your abdominal wall and into your uterus. A small amount of amniotic fluid is then withdrawn into a syringe, and the needle is removed.
- Chorionic villus sampling. For this prenatal genetic test, your doctor takes a tissue sample from the placenta. Depending on your situation, the sample may be taken with a tube (catheter) through your cervix or through your abdominal wall and uterus using a thin needle.
The amount of time it takes for you to receive your genetic testing results will depend on the type of test and your health care facility. Talk to your doctor before the test about when you can expect the results. The lab will likely provide the test results to your doctor in writing. Your doctor can then discuss them with you.
If the genetic test result is positive, that means the genetic alteration that was being tested for was detected. The steps you take after you receive a positive result will depend on the reason you underwent genetic testing. If the purpose was to diagnose a specific disease or condition, a positive result will help you and your doctor determine the right treatment and management plan.
If you were tested to find out if you are carrying an altered gene that could cause disease in your child, and the test is positive, your doctor or a genetic counselor can help you determine a child's risk of actually developing the disease. The test results can also provide information to consider as you and your partner make family planning decisions.
If you were having gene testing to determine if you might develop a certain disease, a positive test doesn't necessarily mean you will get that disorder. For example, having a breast cancer gene (BRCA1 or BRCA2) means you are at high risk of developing breast cancer at some point in your life, but it doesn't indicate that you will get breast cancer. However, there are some conditions, such as Huntington's disease, for which the altered gene does indicate that the disease will eventually develop.
Talk to your doctor about what a positive result means for you. In some cases, you can make lifestyle changes that may decrease your risk of developing a disease, even if you have an altered gene that makes you susceptible to a disorder. Results may also help you make choices related to family planning, careers and insurance coverage.
In addition, you may choose to participate in research or registries related to your genetic disorder or condition. These options may help you stay updated with new developments in prevention or treatment.
A negative result means a genetic alteration was not detected by the test. But a negative result doesn't guarantee that you don't have an alteration. The accuracy of genetic tests to detect alterations varies, depending on the condition being tested for and whether or not an alteration has been previously identified in a family member.
Even if you don't have the genetic alteration, that doesn't necessarily mean you will never get the disease. For example, people who don't have a breast cancer gene (BRCA1 or BRCA2) can still develop breast cancer. Also, genetic testing may not be able to detect all genetic defects.
In some cases, a genetic test may not be able to provide helpful information about the gene in question. Everyone has variations in the way genes appear (polymorphisms) and, often, these variations don't affect your health. But sometimes it can be difficult to distinguish between a disease-causing gene alteration and a harmless gene variation. In these situations, follow-up testing may be necessary.
No matter what the results of your genetic testing, talk with your doctor or genetic counselor about questions or concerns you may have, so you understand what the results mean for you and your family.
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- BRCA1 and BRCA2: Cancer risk and genetic testing. National Cancer Institute. http://www.cancer.gov/cancertopics/factsheet/Risk/BRCA. Accessed Nov. 20, 2010.
- Raby BA, et al. Genetic counseling and testing. http://www.uptodate.com/home/index.html. Accessed Nov. 20, 2010.