Colon cancer genetic testing is a blood test that can tell you whether you carry a rare changed, or mutated, gene 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.

Colon cancer almost always begins as small growths on the inner wall of the colon called 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, doctors usually recommend screening with colonoscopy every 1 to 2 years starting at age 40, or 10 years earlier than the youngest age at which a member of your family was diagnosed with colorectal cancer.¹

The most common genetic changes occur in two conditions: 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.

People with FAP have an almost 100% chance of getting colorectal cancer.^{2, 1} They can develop polyps in their 20s and 30s, or even earlier. People with FAP may decide 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 four or more different genes can cause HNPCC. These changes also increase the risk for other cancers, including cancer of the endometrium, ovaries, and other organs.

Having HNPCC does not mean you definitely will develop colon cancer, but it does increase your chances. If you have one parent who has HNPCC, you have a 50% chance of having one or more of these gene changes. If both parents have it, your chances of getting it are about 80%. (Not every person with the genes will get the condition, and not everyone with the condition will have the abnormal gene.)

If you have a strong family history of colon cancer, you may want to have a blood test to look for changed genes. Testing is most reliable when the family member that has colon cancer, FAP, or HNPCC also tests positive for the abnormal gene. Genetic testing may be recommended if:

• You have at least three relatives who have had colon cancer, and at least one of them 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 got cancer before age 50.