Unlike most other medical tests, genetic tests can reveal information not only about the person being tested but also about that person’s relatives. The presence of a harmful genetic mutation in one family member makes it more likely that other blood relatives may also carry the same mutation. Family relationships can be affected when one member of a family discloses genetic test results that may have implications for other family members. Family members may have very different opinions about how useful it is to learn whether they do or do not have a disease-related genetic mutation. Health discussions may get complicated when some family members know their genetic status while other family members do not choose to know their test results. A conversation with genetics professionals may help family members better understand the complicated choices they may face.


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How is genetic testing done?

Genetic tests are usually requested by a person’s doctor or other health care provider. Although it may be possible to obtain some genetic tests without a health care provider’s order, this approach is not recommended because it does not give the patient the valuable opportunity to discuss this complicated decision with a knowledgeable professional.

Testing is done on a small sample of body fluid or tissue—usually blood, but sometimes saliva, cells from inside the cheek, skin cells, or amniotic fluid (the fluid surrounding a developing fetus).

The sample is then sent to a laboratory that specializes in genetic testing. The laboratory returns the test results to the doctor or genetic counselor who requested the test. In some cases, the laboratory may send the results to the patient directly. It usually takes several weeks or longer to get the test results. Genetic counseling is recommended both before and after genetic testing to make sure that patients have accurate information about what a particular genetic test means for their health and care.

What do the results of genetic testing mean?

Genetic testing can have several possible results: positive, negative, true negative, uninformative negative, false negative, variant of unknown significance, or benign polymorphism. These results are described below.

A “positive test result” means that the laboratory found a specific genetic alteration (or mutation) that is associated with a hereditary cancer syndrome. A positive result may:

  • Confirm the diagnosis of a hereditary cancer syndrome
  • Indicate an increased risk of developing certain cancer(s) in the future
  • Show that someone carries a particular genetic change that does not increase their own risk of cancer but that may increase the risk in their children if they also inherit an altered copy from their other parent (that is, if the child inherits two copies of the abnormal gene, one from their mother and one from their father).
  • Suggest a need for further testing
  • Provide important information that can help other family members make decisions about their own health care.

Also, people who have a positive test result that indicates that they have an increased risk of developing cancer in the future may be able to take steps to lower their risk of developing cancer or to find cancer earlier, including:

  • Being checked at a younger age or more often for signs of cancer
  • Reducing their cancer risk by taking medications or having surgery to remove “at-risk” tissue (These approaches to risk reduction are options for only a few inherited cancer syndromes.)
  • Changing personal behaviors (like quitting smoking, getting more exercise, and eating a healthier diet) to reduce the risk of certain cancers

A positive result on a prenatal genetic test for cancer risk may influence a decision about whether to continue a pregnancy. The results of pre-implantation testing (performed on embryos created by in vitro fertilization) can guide a doctor in deciding which embryo (or embryos) to implant in a woman’s uterus.

Finally, in patients who have already been diagnosed with cancer, a positive result for a mutation associated with certain hereditary cancer syndromes can influence how the cancer is treated. For example, some hereditary cancer disorders interfere with the body’s ability to repair damage that occurs to cellular DNA. If someone with one of these conditions receives a standard dose ofradiation or chemotherapy to treat their cancer, they may experience severe, potentially life-threatening treatment side effects. Knowing about the genetic disorder before treatment begins allows doctors to modify the treatment and reduce the severity of the side effects.

A “negative test result” means that the laboratory did not find the specific alteration that the test was designed to detect. This result is most useful when working with a family in which the specific, disease-causing genetic alteration is already known to be present. In such a case, a negative result can show that the tested family member has not inherited the mutation that is present in their family and that this person therefore does not have the inherited cancer syndrome tested for, does not have an increased genetic risk of developing cancer, or is not a carrier of a mutation that increases cancer risk. Such a test result is called a “true negative.” A true negative result does not mean that there is no cancer risk, but rather that the risk is probably the same as the cancer risk in the general population.

When a person has a strong family history of cancer but the family has not been found to have a known mutation associated with a hereditary cancer syndrome, a negative test result is classified as an “uninformative negative” (that is, does not provide useful information). It is not possible to tell whether someone has a harmful gene mutation that was not detected by the particular test used (a “false negative”) or whether the person truly has no cancer-predisposing genetic alterations in that gene. It is also possible for a person to have a mutation in a gene other than the gene that was tested.

If genetic testing shows a change that has not been previously associated with cancer in other people, the person’s test result may report “variant of unknown significance,” or VUS. This result may be interpreted as “ambiguous” (uncertain), which is to say that the information does not help in making health care decisions.

If the test reveals a genetic change that is common in the general population among people without cancer, the change is called a polymorphism. Everyone has commonly occurring genetic variations (polymorphisms) that are not associated with any increased risk of disease.

Who can help people understand their test results?

A genetic counselor, doctor, or other health care professional trained in genetics can help an individual or family understand their test results. Such counseling may include discussing recommendations for preventive care and screening with the patient, referring the patient to support groups and other information resources, and providing emotional support to the person receiving the results.

In some cases, a genetic counselor or doctor may recommend that other family members consider being tested for specific gene changes that indicate an increased risk of cancer. The decision to test other family members is complicated. It requires a careful evaluation of family history and other factors as well as advice from a genetic counselor or other professional trained in genetics. In general, physicians rely on the family member who has been tested to share the genetic information with their relatives so that family members will know that a genetic condition has been identified in their family. Then, each family member will need to make their own decision regarding whether or not to be tested themselves.