Doctors use genetic tests to obtain information about your health. The broadest definition of genetic testing includes all tests that are ordered to look for evidence of inherited traits or diseases, or diseases caused by a new genetic change that was not inherited. Some genetic tests analyze your chromosomes or DNA, the chemical material you inherited from your parents. Other tests examine the chromosomes or proteins. Genetic tests search for specific changes in DNA or the proteins made from the DNA. Some changes could increase your chance of developing a particular disease. Other changes might not affect you, but could put your children at risk. Your doctor will first ask you what diseases or disorders people in your family (living and dead) have had. The doctor will perform a physical exam and perhaps order some laboratory tests. Armed with this information, you and your doctor can discuss whether genetic testing might be useful or reassuring for you. (This information is especially valuable when planning a pregnancy.)
|What is DNA?|
DNA is the chemical material you inherited from your parents. Your chromosomes are made of long DNA strands. It makes you different from every person who has ever existed (unless you’re an identical twin).
What Happens During A Genetic Test?
It’s simple. Blood is drawn from your arm in the usual way and sent to a special lab. Sometimes, a sample of cells from inside your mouth can be used. In the lab, scientists isolate the DNA from the cells in your blood or study your chromosomes or the proteins they create.
What Do The Tests Look For?
Many genetic tests today involve sequencing the DNA or looking at the chromosomes for missing or extra pieces. Some genetic tests do not involve DNA or chromosome analysis. Commonly ordered tests, such as red blood cell counts and blood levels of liver enzymes, can provide the initial screening tests for some inherited conditions.
Different testing methods are used. Large changes can be seen under a microscope. These changes include an extra copy of a chromosome, as in Down syndrome, or a missing chromosome, as in Turner syndrome.
Extra nucleotides are added to the sequence.
Part of the DNA sequence is missing.
Other tests compare your DNA to known mutations. These mutations can confirm the diagnosis of an existing disease or can increase a person’s chance of developing a disease. In general, the test will only look for “hot spots” on the DNA: particular areas of the chromosomes that research has shown to be involved in disease, such as specific genes. There are probably other mutations that scientists don’t know about yet that lead to disease. These mutations would not be detected in this type of genetic test.
It is more common now for genetic tests to look at the whole gene. This increases the chances of finding a change that has never been seen before. This can make it difficult to know what the change means for you.
Finally, some genetic tests look for changes in proteins, which reflect changes in your DNA. Such tests look for the presence, absence or function of a protein. This information can tell doctors if the gene that makes the protein is working properly. For example, newborn babies are screened for a disease called phenylketonuria, a condition in which an important protein is missing due to a defective gene. Untreated, phenylketonuria can lead to severe mental retardation. The test does not check the gene directly. It looks for high levels of a blood chemical that builds up when the gene isn’t working.
Who Gets Genetic Tests?
There are a number of reasons why your doctor might suggest you have a genetic test.
- If a certain disease, such as breast or ovarian cancer, runs in your family, a genetic test might tell you if you have a higher-than-average chance of developing the disease. This type of testing is called “susceptibility testing.”
- If a parent has a genetic problem such as Huntington’s disease, “predictive testing” can tell you if you have inherited that parent’s normal gene or mutated gene.
- If you already have some symptoms or signs of a genetic disease, a genetic test may confirm or deny the diagnosis. This is called “diagnostic testing.” This is especially helpful when standard evaluation does not provide a definitive answer. One example is fragile X syndrome, a common cause of mental retardation. Without the diagnostic test, it’s hard to differentiate a child with fragile X from any other child with mental retardation.
- If you are thinking about having children, a genetic test can tell you ahead of time if you have a mutation that could increase your child’s chance of developing a genetic disease. This is referred to as “carrier testing.” One example is cystic fibrosis, which affects one in four children born to parents who are both carriers. DNA testing can tell if you are a carrier for this disorder and can help you plan for healthy children. If your ancestors were from a part of the world where certain genetic mutations are known to be common, such as those which cause thalassemia (Mediterranean and Far East Asian areas) or sickle cell disease (Africa) or Tay-Sachs disease (Eastern European Ashkenazi Jewish), carrier testing may be useful.
- If you are pregnant, genetic testing procedures such as amniocentesis, chorionic villus sampling, ultrasound imaging, and DNA testing, as well as screening blood tests for the mother, can help tell if your fetus is developing normally or might be born with certain disorders, such as Down syndrome. This is referred to as “prenatal testing.”
- In a newborn baby, genetic tests can reveal certain disorders, such as phenylketonuria, in time to treat and prevent abnormal mental and physical development. This is called “newborn screening.”
How do I decide if I need a genetic test?
The first step is to learn if you’re at risk for any genetic conditions. Your doctor will ask you what diseases people in your family have, such as breast cancer, Alzheimer’s disease and others. The doctor will then perform a physical exam and, perhaps, order some laboratory tests. Based on this information, your doctor might suspect that you’re at risk for a known hereditary disease. Your doctor will tell you if that disease can be detected with a genetic test. Finally, your doctor will explain the risks and benefits and have you sign a permission form for the test.
How are the tests done?
A health-care professional draws your blood and sends it to a laboratory. At the lab, scientists use DNA from the cells in your blood to perform the genetic test.
Is there a genetic test for every disease?
No. Most diseases do not have a genetic test. Nevertheless, there are hundreds of tests available. These tests can detect more than 1,000 diseases. Some of these diseases can be treated and some cannot.
Why do people get tested for diseases that have no cure?
There are three main reasons:
Sometimes therapies can delay the onset of a disease.
You might want to know for family planning purposes. For example, everyone has two copies of the cystic fibrosis gene, one from mom and one from dad. It takes two changed copies of the cystic fibrosis gene to have the disease. If you have one normal copy and one changed copy, you won’t have any symptoms. However, you could pass on your changed copy of the gene to your child. If the other parent also passes on a changed copy, then your child will have two changed copies and have cystic fibrosis. Some diseases can be detected prior to birth. This gives parents time to decide whether to continue the pregnancy or not and to prepare for the medical needs of the child.
New therapies to treat diseases are being developed at a rapid pace. If you learn you have an increased risk of a certain genetic disease, you and your doctor can watch for news of possible preventions or treatments.
How expensive are the tests?
Genetic tests can range from $200 to $3,000 or more.
Will my insurance cover the cost?
Insurance companies might cover all or a portion of the cost. Many tests are currently not covered. One insurance company, Aetna, is currently leading an initiative with members of the medical profession, academicians, and policy makers in setting guidelines for access to genetic counseling, genetic testing and the appropriate use of test results.
How are tests done?
Most commonly, blood is drawn and sent to a lab, where scientists isolate DNA from cells in your blood and use it to perform the genetic test. Sometimes a lab can use skin cells scraped from the inside of your cheek or collected in a special mouthwash. Your doctor can tell you what type of sample the lab needs.
How long will it take to get my results?
This varies depending on the type of test, which laboratory is doing the test and what gene/disease is being tested. Results of some tests are available in less than a week while others take several months. Most of the time, results are available within 4 to 6 weeks or less.
My test came back positive. Does that mean I will get the disease?
Not necessarily. As with any test, there is always a chance for a “false positive,” meaning that even though the results appear positive, the results do not portray the true picture and the results should have been negative. This is an important risk to think about before getting tested. Some genetic changes appear differently in different people. One person may have no signs of the disease; another family member may be severely affected.
Some types of genetic tests will tell you that you have a 100% chance of having that disease, or of developing it in the future. Some genetic tests can only tell you that you have an above-average chance of developing the disease. For example, a positive result on a BRCA1 test means that you have up to an 80% chance of developing breast or ovarian cancer by age 70. But you also have a 20% chance of not developing breast cancer. Genetic tests cannot predict when you will get the disease — you might be 85 before you develop any signs of breast cancer.
My test came back negative. Does this mean I do not have or will not get the disease?
Not necessarily. Just as there are false positives, there are also “false negatives.” Although your test results came back negative, your DNA might really contain a mutation that will increase your chance of developing the disease. The tests only search for the most common disease-causing mutations, and scientists don’t know all of the mutations that lead to any given disease.
I have a disease that can be identified with a genetic test. I’m wondering if my daughter has the same mutation. Should I have her tested?
One thing to consider is whether this information will help your daughter’s doctor manage her health. If it won’t make a difference, then perhaps you should wait until she can decide for herself if she wants this testing. For example, if you have the BRCA1 breast-cancer mutation, your daughter’s doctor will not need to know if she is also positive for this mutation when she’s only 5 years old because it wouldn’t affect her health at that age. (She will not be at risk for developing breast cancer for many years.) A positive test result could also limit your child’s ability to eventually obtain life and disability insurance. On the other hand, if there are actions that could delay the onset of the disease, genetic testing might be worthwhile. Talk this over with your daughter’s doctor, a medical geneticist (physician trained in genetic medicine) or a genetic counselor (counselor with training in human genetics).
I have a genetic disease. What are the chances that my child will have the same disease?
It depends on the disease. Some diseases require two copies of a mutated gene (one from mom and one from dad) before a person shows any symptoms. These genes are called “recessive.” Other diseases only require one mutated copy of the disease gene for the person to show signs of the disease. These are called “dominant” genes because the one mutated gene dominates over the good gene. Other diseases are complicated, and require two or more mutated genes before a person has symptoms. It can be very difficult to predict the odds of your child’s inheriting these types of diseases.
My first child has a genetic disease. What are the chances that a future child could inherit the same disease?
The odds that future children will inherit the same disease as your first child will depend on what the disease is, how it is inherited, and the genetic background and physical features of the parents. There is no simple answer that fits all genetic diseases. For dominant or recessive diseases, the risk is more straightforward. For more complex genetic disorders such as heart defects at birth and cleft palate, a genetics doctor may need to examine both parents and the affected child in order to predict whether other children might be affected by the same condition.