Colon cancer diagnosis & tests
Colonoscopies, blood tests, imaging tests and genetic tests are used to diagnosis colon cancer and find out how far the cancer has spread.
Medical review by Mayo Clinic Staff
Updated: April 14, 2025
Colon cancer diagnosis often starts with a colonoscopy. A healthcare professional might do a colonoscopy as part of a colon cancer screening. Or you might have this test if you have symptoms that concern you. If the colonoscopy finds something out of the ordinary, you may have further testing to look for colon cancer.
Once the diagnosis of colon cancer is made, additional tests are done to find out how far the cancer has spread. This is called staging. Colon cancer stages range from stage 0 to stage 4.
Colon cancer screening tests
Colon cancer screening tests look for colon cancer in people who don’t have any symptoms of the disease. These tests typically include a colonoscopy or noninvasive tests that can be done at home. Colon cancer screening is recommended at age 45 for adults at average risk. Those at higher risk due to family history or certain medical conditions may start screening earlier.
Colonoscopy
Colonoscopy uses a long, flexible tube attached to a video camera to view the whole colon and rectum on a video monitor. A healthcare professional may pass surgical tools through the tube to take tissue samples, called a biopsy, and to remove polyps.
During a colonoscopy, a healthcare professional puts a colonoscope into the rectum to check the entire colon.
“A colonoscopy is not just diagnostic but can also be therapeutic. By removing the highly aggressive polyps, or at-risk polyps, you’ve technically removed the risk of colon cancer.”
Tanios S. Bekaii-Saab, M.D.
Mayo Clinic oncologist
Biopsy
A biopsy is a procedure to remove a sample of tissue for testing in a lab. For colon cancer, the tissue sample often is collected during a colonoscopy. Sometimes surgery is needed to get the tissue sample. In the lab, tests can show whether the cells are cancerous and how quickly they’re growing. Other tests can give more information about the cancer cells. Your healthcare team uses the results to understand your prognosis and create a treatment plan.
Stool tests
Stool tests are noninvasive screening methods used to find colon cancer. These tests are generally easy to do at home and may be recommended for people at average risk of colon cancer. Types of stool tests include:
- Fecal occult blood test. This test looks for blood in a sample of stool. There are two different types: the guaiac-based fecal occult blood test and the fecal immunochemical test. These tests are called gFOBT and FIT for short. It’s recommended that a fecal occult blood test be done once a year. When done every year, the test is more than 90% effective at detecting colon cancer. But the test may miss some cancers or polyps that don’t bleed.
- Stool DNA test. This test uses a stool sample to look for certain DNA changes that happen with colon cancer. One stool DNA test (Cologuard) is approved for colon cancer screening in the United States. For most people, the United States Preventive Services Task Force recommends stool DNA testing every 1 to 3 years. If done once a year, stool DNA testing is almost 90% effective at detecting colon cancer.
It’s important to note that if any stool test result is positive, a follow-up colonoscopy is needed to confirm the findings and remove any precancerous polyps. Stool testing generally is not used to test for colon cancer in people who have colon cancer symptoms or a personal or family history of colon cancer or certain genetic syndromes that increase the risk of cancer.
Shield liquid biopsy
Recently, the Food and Drug Administration approved a new blood test called Shield. Shield is recommended as a colon cancer screening test only for people at average risk. This test detects specific DNA changes in the blood that indicate cancer or precancerous growths. In a study of nearly 8,000 people, Shield identified over 83% of colon cancers but was less effective at detecting precancerous growths, with a sensitivity of only about 13%.
Genetic testing
Genetic testing isn’t used for screening or diagnosing colon cancer, but it can offer valuable information about your risk. This testing involves examining a blood or saliva sample for specific genes that may put you at high risk of colon cancer. This information can guide treatment decisions and also let family members know about their possible risk.
Genetic testing is recommended for people who:
- Have a strong family history of colon cancer or polyps.
- Have colon cancer along with other types of cancer.
- Have family members with a condition that increases colon cancer risk. These include Lynch syndrome, familial adenomatous polyposis, which also is called FAP, and MUTYH-associated polyposis, also called MAP.
- Have relatives with colon cancer or related cancers, including rectal, endometrial or uterine cancer.
- Have been diagnosed with colon cancer before age 50.
Imaging tests to look for colon cancer that has spread
Imaging tests can look for areas of cancer inside the body. CT scanning is the main imaging test used to detect colon cancer. CT scanning also can see how far the cancer has spread if you’ve already been diagnosed with colon cancer.
CT scanning may be combined with PET scanning to help care teams decide if surgery is an option for cancer that has spread outside the colon. Other imaging tests, such as MRI scanning, X-rays or ultrasound, may be done to help care teams make decisions about a surgical approach.
Other tests
While these tests may not diagnose colon cancer, they can provide valuable insight into how your body is coping with the disease.
Tumor marker testing
Colon cancers sometimes make proteins called tumor markers in response to cancer. Common tumor markers for colon cancer include carcinoembryonic antigen. This marker also is called CEA. It is often used to monitor treatment response and detect recurrence. CA 19-9 is another common tumor marker, and it may be elevated in some cases. Blood tests can track the level of CEA and CA 19-9 over time. The results might show whether the cancer is responding to treatment. After treatment, blood tests for these tumor markers might detect if the cancer comes back.
Blood analysis
Looking at a blood sample can give clues about overall health, such as how well the kidneys and liver are working. A blood test might be used to look for a low level of red blood cells. This result might indicate that a colon cancer is causing bleeding.
Your healthcare team uses the results of your tests and procedures to give your cancer a stage. The cancer’s stage tells your healthcare team about the size of the cancer and how aggressive the treatment needs to be. Your healthcare team considers the cancer’s stage when creating a treatment plan.
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Mayo Clinic’s approach to diagnosing colon cancer
At Mayo Clinic, we believe the best colon cancer screening is the one you will actually do. That’s why we helped invent minimally invasive screening options, and it’s why we keep researching the impact of early detection for colon cancer. Finding cancer early, when it is easier to treat, gives people more options, more time and a better chance at a cure.
The colon cancer experts at Mayo Clinic understand this because they have seen it happen. The latest testing technology in the hands of our highly skilled experts helps more than 4,000 patients get the colon cancer care they need at Mayo Clinic each year.
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