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.
Support group discussions
From Mayo Clinic Connect, our online patient community.
Find more discussions:
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.
Colon cancer can be diagnosed during a routine screening that is done when you do not have any symptoms or through testing that is done because you are having concerning symptoms. It is typically diagnosed by having a biopsy during a colonoscopy.
If you have been diagnosed with colon cancer, you likely will need additional testing to determine the best treatment plan based on the size, location and stage of your cancer. Treatment plans can vary. That's why it's important to receive an accurate and detailed diagnosis. At Mayo Clinic, we know that having the information you need can help you make decisions with confidence. So when you need more information about colon cancer diagnostics, we're here to help.
Screening for colon cancer
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 several types of noninvasive tests are available to choose from for testing at home. If you have an average risk of colon cancer, it is recommended to begin screening at 45 years of age. If you have a higher risk of colon cancer due to family history or certain medical conditions, you may need to start screening earlier.
There are many types of colon cancer screening. Your healthcare professional can help you understand which test is the right one for you.
Colonoscopy
During a colonoscopy, your healthcare professional uses a long, flexible tube attached to a video camera to view the whole colon and rectum on a video monitor. Your healthcare professional may use specialized tools that are passed through the tube to take tissue samples and take out polyps.
A colonoscopy can be used both as a screening and a diagnostic tool. The colon cancer team at Mayo Clinic is studying ways to enhance colonoscopies with artificial intelligence (AI). AI-assisted colonoscopies can help healthcare professionals detect and characterize polyps, ensuring care teams find and remove all irregular cells as early as possible.
While a colonoscopy can help you understand if any concerning or irregular cells were found in your colon, you'll need additional testing to confirm that those cells are cancerous. If your colonoscopy results are not typical, the team at Mayo Clinic is here to help. Our colon cancer team treats thousands of patients with colon cancer each year. And because team members are so experienced with colon cancer, they are very skilled at helping patients understand their colonoscopy results and arrive at a diagnosis that helps explain what the results mean.
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
Stool tests
While a colonoscopy is the most common colon cancer screening, the team at Mayo Clinic believes the best colon cancer screening is the one that you get done. That's why Mayo Clinic has been involved with research that advances noninvasive colon cancer screening options.
- Stool DNA test. This test uses a stool sample to look for certain DNA changes that happen with colon cancer. One stool DNA test, commonly called Cologuard, is approved for colon cancer screening in the United States. Mayo Clinic's research teams were instrumental in this approval process. 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.
- 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 (gFOBT) and the fecal immunochemical test (FIT). 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 tests can be an effective screening method. If you have a stool test that is positive, the next step is to have a colonoscopy. Most of the time a positive stool test does not mean that you have cancer. But it does mean that it is important to have a colonoscopy to find out if you might have a precancerous polyp, a cancer or a healthy colon.
If you have an average risk of colon cancer, stool testing may be a good option for you. Stool tests are approved for use in people at average risk of colon cancer. Currently, stool testing is not recommended if you have colon cancer symptoms, a personal or family history of colon cancer, or certain genetic syndromes that increase your risk of cancer.
Cell-free DNA test
The Food and Drug Administration recently approved a new blood test for colon cancer called a cell-free DNA test or a liquid biopsy. If you have an average risk of colon cancer, this blood test may be a good option for you. The test detects specific DNA changes in the blood that indicate cancer or precancerous growths. These changes are called cell-free DNA because they float in the blood.
Mayo Clinic was part of a large study leading to the approval of one cell-free DNA test called Shield or liquid biopsy. The study included nearly 8,000 people and showed that the test identified over 83% of colon cancers. It was less effective at detecting precancerous growths, with a sensitivity of only about 13%.
Diagnosing colon cancer
You may receive an initial colon cancer diagnosis following a colonoscopy, but you likely will need additional tests to confirm your diagnosis. During a colonoscopy, your healthcare professional can remove some of the irregular cells to then test the cells in a lab. If the cells are cancerous, your care team may recommend an initial treatment plan that includes surgery. You may not know the details about your cancer's stage until after you have had surgery to remove the cancer.
Biopsy
A biopsy is a procedure to remove a sample of tissue for testing in a lab. This tissue sample often is collected during a colonoscopy. Sometimes the area where cancer is suspected is not reachable during a colonoscopy. This may mean that tissue has to be obtained another way. An interventional radiologist can collect tissue using a special imaging test, such as ultrasound or CT scan, to guide where to do a needle biopsy. Or, more rarely, surgery may be needed to get the tissue sample.
In the lab, tests can show whether the cells are cancerous. Other tests can give more information about the cancer cells that may help guide treatment and even help determine if you might have a genetic risk of colon cancer. Your healthcare team uses the results to understand your prognosis and create a treatment plan.
Support group discussions
From Mayo Clinic Connect, our online patient community.
Find more discussions:
Colon cancer tests that may support your care plan
In addition to a biopsy, there are other tests that can help your team better understand your cancer or risk of cancer. These tests allow you and your team to make treatment decisions with confidence. At Mayo Clinic, we believe that the best care plans are personalized to you, your cancer and your unique health history. That's why many of the following tests are part of our patients' care plans.
Two types of genetic testing
If you have colon cancer, genetic testing of your tumor tissue can help identify the right treatments for your specific cancer. Changes in the mismatch repair genes, such as MLH1, may be used to identify if your colon cancer is more likely to respond to treatment with immunotherapy. It also may indicate a hereditary condition called Lynch syndrome. Other genetic changes in cancer tissue may indicate that the tumor may or may not respond to a specific type of chemotherapy.
There is another type of genetic testing that isn't used for screening or diagnosing colon cancer, but it can offer valuable information about your cancer risk. This genetic information can help you understand your cancer risk so that you can follow a personalized screening plan. If you're diagnosed with cancer, experts at Mayo Clinic use this type of genetic information to offer personalized treatment options. Your genetic information also can help you let family members know about their possible risk. During a genetic test, a sample of your blood or saliva is examined for specific genes that may put you at a high risk of cancer.
Genetic testing is recommended if you:
- 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 conditions include Lynch syndrome, familial adenomatous polyposis (FAP) and MUTYH-associated polyposis (MAP).
- Have relatives with colon cancer or related cancers, including rectal, endometrial or uterine.
- Have been diagnosed with colon cancer before age 50.
Imaging tests to look for colon cancer that has spread
If you're diagnosed with colon cancer, imaging tests can help you and your care team find out if your cancer has spread. CT scanning is the main imaging test used to detect colon cancer and see how far the cancer has spread.
If your cancer has spread outside of the colon, CT scanning may be combined with PET scanning to help your care team decide if surgery is an option for you. Other imaging tests, such as MRI scanning, X-rays or ultrasound, also may help you and your care team make decisions about your surgical plan.
Tumor marker testing
Colon cancer sometimes makes proteins called tumor markers. Carcinoembryonic antigen (CEA) is a common colon cancer tumor marker. Your care team may test for CEA to monitor treatment response or detect recurrence. Blood tests can track your CEA and cancer antigen 19-9 (CA 19-9) over time to show whether your cancer is responding to treatment. After treatment, blood tests for these tumor markers might detect if the cancer has returned.
Blood analysis
Looking at a blood sample can give clues about your overall health, such as how well your 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 your 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 provides information 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.
Mayo Clinic’s approach to diagnosing colon cancer
At Mayo Clinic, we believe the best colon cancer screening is the one you will actually get done. 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.
You can also contact our helpful scheduling teams by calling:
Associated procedures
News from Mayo Clinic
- Mayo Clinic Minute: Liver transplant for patients with advanced colorectal cancer
- Liver transplants offer new hope for colorectal cancer patients with liver metastasis
- Mayo Clinic Q and A: Colorectal cancer in young adults
- You’re at risk of colorectal cancer. Know the symptoms. Get screened.
- (VIDEO) Treatment options for advanced colorectal cancer
- Mayo Clinic Minute: Warning signs of colorectal cancer in younger adults
- Mayo Clinic researchers identify proteins linked to immunotherapy resistance in metastatic colorectal cancer
- Mayo Clinic Minute: Are colon and rectal cancers treated differently?
- Mayo Clinic Q and A: Understanding colorectal cancer
- Clinical trials team brings hope and dedication to colorectal cancer study
- New gene markers detect Lynch syndrome-associated colorectal cancer, Mayo Clinic study
- Mayo Clinic researchers publish key findings about cell proteins to determine effectiveness of immunotherapy for colon cancer
