Discusses blood test that can tell you if you carry rare changed genes that can cause colon cancer, also called colorectal cancer. Covers familial adenomatous polyposis (FAP). Looks at colonoscopy screening. Discusses risks. Includes genetic counseling.
Colon Cancer Genetic Testing
What Is Colon Cancer Genetic Testing?
Colon cancer genetic testing is a blood test that can tell you
whether you carry rare changed, or mutated,
genes that can cause
colon cancer. Although most people who get colon
cancer do not have one of these mutated genes, having them greatly increases
your chance of getting colon cancer.
Colon cancer develops in the
large intestine when cells change and grow out of control. Colon cancer is
also called colorectal cancer, because it can occur in both the colon and in the
lowest section of the colon, which is called the rectum.
cancer almost always begins as small growths on the inner wall of the colon
polyps. A doctor can find and remove polyps during a
colonoscopy, a test in which a doctor uses a flexible
video camera or scope to look at the inside of the colon. If a close member of
your family, such as your brother, sister, or parent, has had colon cancer,
talk with your doctor about colonoscopy screening.
The most common
genetic changes related to colon cancer are familial adenomatous polyposis
(FAP) and hereditary nonpolyposis colorectal cancer (HNPCC). In these
conditions, screening often starts even sooner than age 40.
Familial adenomatous polyposis (FAP)
FAP develops because of a changed gene that causes
hundreds or thousands of polyps to grow in the colon. The number of polyps
increases with age. If one of your parents has FAP, you have a 50% chance of
having the changed gene and the disease.
Everyone who has FAP will get colorectal cancer if they are not treated. They can develop polyps in their
20s and 30s, or even earlier. People who have FAP need to have their colons
removed to prevent colon cancer.
Hereditary nonpolyposis colorectal cancer (HNPCC) or Lynch syndrome
HNPCC can cause polyps in the colon, but not as many
as FAP. Changes in any of one or more different genes can cause HNPCC. These
changes also increase the risk of getting other cancers, including cancer of the
endometrium, ovaries, stomach, urinary tract, small intestine, skin, brain, and liver.
Having the gene change related to HNPCC does not mean
you definitely will develop colon cancer. But it does increase your chances of getting colon cancer at a younger age. And the cancer is more likely to grow faster. It also increases your chances of other cancers, so screening for endometrial or other cancers may be important. If a person has an HNPCC gene change, he or she has a 50% chance of passing this gene on to a child.
If you have a very strong family history of colon cancer,
you may want to have a blood test to look for changed genes. These things are common in families with HNPCC:
You have at least three relatives who have had
colon cancer, endometrial cancer, or another HNPCC-related cancer, and at least one of the relatives is a parent, brother, or sister.
Those relatives are spread over two generations in a row (for
example, a grandparent and a parent).
One of those relatives had
colorectal or endometrial cancer before age 50.
Testing is more useful when the family member who has
colon cancer, FAP, or HNPCC also tests positive for the abnormal gene. If your relative who has the condition does not test positive, this means he or she does not have the changed gene. If your relative doesn't have the changed gene, most likely you don't either, so you would not need to be tested for this changed gene. But you would still have a high risk. This means you would need to be watched closely by your doctor and have careful screening.
What Do the Results Mean?
Colon cancer genetic
testing is a blood test that looks for the changed (mutated) genes that cause
colon cancer. Although most people who get colon cancer do not have one of
these mutated genes, having them greatly increases your chance of getting colon
A positive result means that you may have one of the
changed genes that causes FAP or HNPCC. It also means that, without treatment, your chances of
getting colon cancer are very high.
If you have a positive result, you will be able to take action that may help you and your family members live longer. This includes starting screening at a younger age and having screening tests more often.
A negative result means that none of these
genes were found in your blood sample.
How Accurate Is the Test?
Although these blood tests
are highly reliable, no test is 100% accurate. The test cannot tell you when or
whether you will develop colon cancer. Testing negative for an inherited colon
cancer gene does not mean you will never get colon cancer. It means your risk
of colon cancer is about the same as that of the average person.
There are many different genetic mutations that can lead to FAP or HNPCC.
Genetic testing does not find all of them. Because of this, a person may have a
normal genetic test but still have an increased chance of having an inherited
Should I Be Tested?
The decision to be tested for
genetic colon cancer is personal. You may have emotional, financial, and family
reasons for taking or not taking the test. Also, there is the possibility that if you do test positive, you may have difficulty getting life insurance, long-term care insurance, or disability insurance.
You might choose to be
You have a family history of colon cancer. This means you have a parent, brother, sister, or child who has colon cancer, FAP, or HNPCC and they have tested positive for
the abnormal gene.
You have a personal history of more than 20 colon
polyps, especially at a young age.
You have received
genetic counseling, understand the risks and benefits
of testing, and feel that the benefits outweigh the risks. A
genetic counselor can help you make well-informed
What Is Genetic Counseling?
Information from genetic
testing can have a profound impact on your life. Genetic counselors are trained
to explain the test and its results, but you make the decision about whether to
have the test. A genetic counselor can help you make well-informed decisions.
Ask to have genetic counseling before making a decision about testing. Genetic
counseling can help you and your family:
Understand medical facts, including what causes diseases, how a
diagnosis is made, and what you may be able to do to help you manage a
Understand how your family history contributes to the
development of a disease.
Understand what you can do to help
prevent colon cancer or a recurrence.
Learn about caring for a
family member who has a genetic disease, including getting referrals to
specialists or joining support groups.
Genetic counselors are trained to help you and your family
make informed decisions. They are sensitive to the physical and emotional
aspects of these decisions. Your privacy and confidentiality are carefully
Other Places To Get Help
1-888-651-3038 (571) 483-1780
Cancer.Net is the information website of the American
Society of Clinical Oncology (ASCO) for people living with cancer and for those
who care for them. ASCO is the world's leading professional organization
representing physicians of all oncology subspecialties. Cancer.Net provides
current oncologist-approved information on living with cancer.
National Human Genome Research Institute, National
Institutes of Health (NIH)
NIH Building 31, Room 4B09, 31 Center Drive
MSC 2152, 9000 Rockville Pike
Bethesda, MD 20892-2152
Phone: (301) 402-0911
The National Human Genome Research Institute (NHGRI) supports
genetic and genomic research, investigation into the ethical, legal, and social
implications surrounding genetics research, and educational outreach
activities. (Genome refers to a complete gene sequence in a organism.) NHGRI
also supports the training of investigators and provides genome information,
including fact sheets and school materials, to the public and to health
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How this information was developed to help you make better health decisions.