4 Types of Genetic Testing Explained: An Expert's Guide

Clinical genetic testing is different from direct-to-consumer genetic testing (DTC), which can provide information about medical and non-medical characteristics. A doctor will request clinical genetic testing for a specific medical reason, while DTC tests are usually purchased by healthy people who are interested in learning more about traits such as ancestry, responses to medications, or the risk of developing certain complex conditions. The results of the DTC test can be used to make lifestyle decisions or provide topics to discuss with the doctor, but they cannot determine with certainty if you will get a disease or not and should not be used alone to make decisions about your treatment or medical care. As the number of genetic testing has rapidly expanded over the past decade, so have the different types of genetic testing methodologies used.

The type of test used will depend on the type of anomaly being measured. In general, there are three categories of genetic tests available: cytogenetic tests, biochemical tests, and molecular tests to detect abnormalities in chromosomal structure, protein function, and DNA sequence, respectively. Newborn screening tests are performed right after birth to identify genetic disorders that can be treated early in life. For example, all babies in the United Kingdom are tested for cystic fibrosis as part of the heel prick test.

Diagnostic tests are used to identify or rule out a specific genetic disorder if a baby or person has symptoms that suggest a certain genetic disorder (for example, Down syndrome). Carrier testing is used to identify people who carry one copy of a genetic mutation (a genetic change) that, when present in two copies, causes a genetic disorder (such as sickle cell anemia). This type of test can be useful in providing information about a couple's risk of having a child with a genetic disorder. Predictive testing is used to detect genetic mutations associated with disorders that appear after birth, often later in life.

These tests may be useful for people who have a family member with a genetic disorder, but who has no characteristics of the disorder itself at the time of the test (for example, breast cancer associated with the BRCA1 gene). Predictive testing can identify mutations that increase a person's risk of developing genetically based disorders, such as certain types of cancer. Therefore, before undergoing predictive testing, it is essential that a specialist carefully discuss with you your risks of being affected by the disorder, how the disorder would affect you, and the benefits and risks of having a genetic test to detect the disorder. See the section on genetic counseling below.

People from families at high risk of suffering from a genetic disease have to live with uncertainty about their future and the future of their children. If you have a family history of a genetic condition, have symptoms of a genetic condition, or are interested in knowing your chances of having a genetic condition, talk to your doctor to find out if genetic testing is right for you. A variety of genetic tests can be purchased individually, many of them now on the Internet. These usually consist of scraping the inside of the cheek to obtain some cells for analysis.

In newborns, routine tests for a genetic disorder, such as phenylketonuria, are done as part of the baby's heel prick test when they are about 5 days old. The health care provider will consider several factors when selecting the right test, including what condition or conditions are suspected and the genetic variations that are commonly associated with those conditions. These types of tests determine the order of the basic components of DNA (nucleotides) in a person's genetic code through a process called DNA sequencing. Panel genetic tests can also be grouped into genes related to an increased risk of developing certain types of cancer such as breast or colorectal (colon) cancer.

Many tests are done to detect genetic disorders for which there is no treatment; these can increase anxiety if the test tests positive for one of these disorders. Genetic test results can confirm or rule out a possible genetic condition or help determine the chances of a person developing or transmitting a genetic disorder. Large-scale genetic testing can yield results unrelated to the reason the test was requested in the first place (secondary findings). The Genetic Testing Registry is available at the National Center for Biotechnology and Information at the National Library of Medicine.

Genetic counseling before and after genetic testing can help ensure that you are the right person in your family to have a genetic test; that you are doing the right type; and that you understand your results. These types of tests are offered during pregnancy if there is an increased risk of the baby having a genetic or chromosomal disorder. Some genetic tests can be done from samples of saliva or from a sample (swab) from inside the mouth.