Genetic testing involves examining your DNA, the chemical database that carries instructions for your body’s functions. Genetic testing can reveal changes (mutations) 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, a medical geneticist or a genetic counsellor about what you will do with the results is an important step in the process of genetic testing.
When genetic testing doesn’t lead to a diagnosis, but a genetic cause is still suspected, some facilities offer genome sequencing — a process for analysing a sample of DNA taken from your blood.
Everyone has a unique genome, made up of the DNA in all of a person’s genes. This complex testing can help identify genetic variants that may relate to your health. This testing is usually limited to just looking at the protein-encoding parts of DNA called the exome.
Why it’s done
Genetic testing plays a vital role in determining the risk of developing certain diseases as well as screening and sometimes medical treatment. Different types of genetic testing are done for different reasons:
• Diagnostic testing. If you have symptoms of a disease that may be caused by genetic changes, sometimes called mutated genes, genetic testing can reveal if you have the suspected disorder. For example, genetic testing may be used to confirm a diagnosis of cystic fibrosis or Huntington’s disease.
• Presymptomatic and predictive testing. If you have a family history of a genetic condition, getting genetic testing before you have symptoms may show if you’re at risk of developing that condition. For example, this type of test may be useful for identifying your risk of certain types of colorectal cancer.
• Carrier testing. If you have a family history of a genetic disorder — such as sickle cell anaemia or cystic fibrosis — or you’re in an ethnic group that has a high risk of a specific genetic disorder, you may choose to have genetic testing before having children. An expanded carrier screening test can detect genes associated with a wide variety of genetic diseases and mutations and can identify if you and your partner are carriers for the same conditions.
• 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’re pregnant, tests can detect some types of abnormalities in your baby’s genes. Down syndrome and trisomy 18 syndrome are two genetic disorders that are often screened for as part of prenatal genetic testing. Traditionally this is done looking at markers in blood or by invasive testing such as amniocentesis. Newer testing called cell-free DNA testing looks at a baby’s DNA via a blood test done on the mother.
• Newborn screening. This is the most common type of genetic testing. In the United States, all states require that newborns be tested for certain genetic and metabolic abnormalities that cause specific conditions. This type of genetic testing is important because if results show there’s a disorder such as congenital hypothyroidism, sickle cell disease or phenylketonuria (PKU), care and treatment can begin right away.
• Preimplantation testing. Also called preimplantation genetic diagnosis, this test may be used when you attempt to conceive a child through in vitro fertilization. The embryos are screened for genetic abnormalities. Embryos without abnormalities are implanted in the uterus in hopes of achieving pregnancy
Generally genetic tests have little physical risk. Blood and cheek swab tests have almost no risk. However, prenatal testing such as amniocentesis or chorionic villus sampling has a small risk of pregnancy loss (miscarriage).
Genetic testing can have emotional, social and financial risks as well. Discuss all risks and benefits of genetic testing with your doctor, a medical geneticist or a genetic counsellor before you have a genetic test.
How you prepare
Before you have genetic testing, gather as much information as you can about your family’s medical history. Then, talk with your doctor or a genetic counsellor about your personal and family medical history to better understand your risk. Ask questions and discuss any concerns about genetic testing at that meeting. Also, talk about your options, depending on the test results.
If you’re being tested for a genetic disorder that runs in families, you may want to consider discussing your decision to have 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 may affect them.
Not all health insurance policies pay 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 Non-discrimination Act of 2008 (GINA) helps prevent health insurers or employers from discriminating against you based on test results. Under GINA, employment discrimination based on genetic risk also is illegal. However, this act does not cover life, long-term care or disability insurance. Most states offer additional protection.
What you can expect
Depending on the type of 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 takes 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. For some tests, a swab sample from the inside of your cheek is collected for genetic testing.
• Amniocentesis. In this prenatal genetic test, your doctor inserts a thin, hollow needle through your abdominal wall and into your uterus to collect a small amount of amniotic fluid for testing.
• 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 test results depends on the type of test and your health care facility. Talk to your doctor, medical geneticist or genetic counsellor before the test about when you can expect the results and have a discussion about them.
If the genetic test result is positive, that means the genetic change that was being tested for was detected. The steps you take after you receive a positive result will depend on the reason you had genetic testing.
If the purpose is to:
• Diagnose a specific disease or condition, a positive result will help you and your doctor determine the right treatment and management plan.
• Find out if you are carrying a gene that could cause disease in your child, and the test is positive, your doctor, medical geneticist or a genetic counsellor can help you determine your 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.
• Determine if you might develop a certain disease, a positive test doesn’t necessarily mean you’ll get that disorder. For example, having a breast cancer gene (BRCA1 or BRCA2) means you’re at high risk of developing breast cancer at some point in your life, but it doesn’t indicate with certainty that you’ll get breast cancer. However, with some conditions, such as Huntington’s disease, having 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 reduce your risk of developing a disease, even if you have a gene that makes you more susceptible to a disorder. Results may also help you make choices related to treatment, 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 mutated gene was not detected by the test, which can be reassuring, but it’s not a 100 percent guarantee that you don’t have the disorder. The accuracy of genetic tests to detect mutated genes varies, depending on the condition being tested for and whether or not the gene mutation was previously identified in a family member.
Even if you don’t have the mutated gene, that doesn’t necessarily mean you’ll never get the disease. For example, the majority of people who develop breast cancer don’t have a breast cancer gene (BRCA1 or BRCA2). Also, genetic testing may not be able to detect all genetic defects.
In some cases, a genetic test may not provide helpful information about the gene in question. Everyone has variations in the way genes appear, and often these variations don’t affect your health. But sometimes it can be difficult to distinguish between a disease-causing gene and a harmless gene variation. These changes are called variants of uncertain significance. In these situations, follow-up testing or periodic reviews of the gene over time may be necessary.
No matter what the results of your genetic testing, talk with your doctor, medical geneticist or genetic counsellor about questions or concerns you may have. This will help you understand what the results mean for you and your family.