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Healthcare provider speaks with patient about diagnosis of colon cancer.

Colon cancer begins in the colon or rectum. In many cases, it develops from precancerous polyps, which are abnormal growths in the lining of the colon or rectum. If left untreated, colon polyps can progress and form tumors. Common symptoms include changes in bowel habits, such as frequent diarrhea or constipation, rectal bleeding, bloody stool, gas and abdominal pain and bloating.

Routine screening tests, such as periodic colonoscopies, can detect precancerous polyps as well as early-stage colon cancer when it is highly treatable and often curable. Depending on the stage and location of the tumor, the treatment options may include surgery, chemotherapy and radiation therapy.

As with any type of cancer, an early diagnosis of colon cancer is the key to achieving the best possible outcome and quality of life. The diagnostic process may include:

Laboratory tests used for diagnosing colon cancer

If colon cancer is suspected based on a screening test result, medical history review, symptom evaluation and/or physical examination, which may include a digital rectal exam (DRE), a physician may order follow-up lab testing, such as:

Fecal occult blood test (FOBT)

FOBT is a simple, noninvasive screening test that can detect hidden (occult) blood in the stool, which can be a sign of colorectal cancer as well as certain benign conditions, such as hemorrhoids. The test involves analyzing a small stool sample for the presence of blood. Typically, the sample is collected at home with a special kit provided by a physician and then sent to a lab for analysis. When the sample is placed on a test card coated with a chemical substance (guaiac), the card will change color if a trace amount of blood is present.

While FOBT is a useful tool for detecting occult blood in the stool, it is not a definitive test for diagnosing colorectal cancer. A positive result may indicate the need for further testing, such as a diagnostic colonoscopy, to help the physician determine the cause of the bleeding.

Fecal immunohistochemistry test (FIT)

Like FOBT, FIT is a simple, noninvasive screening test that can detect traces of blood in a stool sample collected at home. However, unlike FOBT—which uses a chemical reagent to detect blood—FIT uses antibodies that specifically react to human hemoglobin, the oxygen-transport protein found in red blood cells.

FIT is more sensitive than FOBT and can detect a lower level of blood in the stool. Even so, a FIT test alone cannot be used to conclusively diagnose colon cancer.

Molecular testing

Molecular testing is often performed on a sample of tumor tissue obtained during a biopsy or surgery. By analyzing and interpreting the genetic information contained in the sample, a pathologist can identify specific gene mutations that drive the growth of the cancer cells, such as mutations in the KRAS, NRAS and BRAF genes.

Often used to guide treatment decisions, molecular testing can help a physician determine whether the patient may benefit from targeted therapies, which specifically target mutations in the KRAS, NRAS and BRAF genes, or immunotherapy, which can help the immune system recognize and attack cancer cells.

Imaging tests used for diagnosing colon cancer

During the diagnostic process, a physician may order imaging tests to evaluate the location, size and extent of a tumor. Some options include:

Computed tomography (CT)

During a CT scan, an X-ray beam moves in a circle around the entire body, capturing multiple images of the same internal structures from various angles. The X-ray data is then processed by a computer to create detailed cross-sectional images of the colon and surrounding tissues.

Magnetic resonance imaging (MRI)

During an MRI scan, the patient lies on a special table that slides into a cylindrical machine, where radio waves and a powerful magnet will be used to create detailed cross-sectional images of the colon. Often performed in combination with other imaging tests, MRI can be particularly helpful for evaluating the rectum because it can provide detailed images of the rectal wall and nearby lymph nodes.

Virtual colonoscopy

After inserting a small tube into the rectum, a physician will inflate the colon with air or carbon dioxide for heightened image clarity. Next, the physician will use a CT scanner to capture multiple X-ray images of the colon from various angles. The X-ray data will then be processed by a computer to create detailed 3D images of the colon and surrounding tissues.

Barium enema

During a barium enema, a physician will place a liquid containing barium—a chalky substance that can make the colon and rectum more visible on X-rays—into the colon via the rectum. Once the barium is in place, a radiology technician will capture a series of X-ray images of the colon and rectum.

While barium enema can provide valuable information about the condition of the colon, it is not as sensitive or specific as other imaging tests, such as CT colonography or traditional colonoscopy, for detecting colon cancer. However, if colonoscopy is not feasible, barium enema may be used as an alternative in combination with other diagnostic tests.

Endoscopic ultrasound (EUS)

During an EUS procedure, a physician will insert an ultrasound probe (endoscope) through the rectum and guide it into the colon. The probe will then emit high-frequency sound waves, which will bounce off the internal structures to create a highly detailed image (sonogram).

Procedures used for diagnosing colon cancer

Common diagnostic procedures for colon cancer include:


During a colonoscopy, a physician will insert a long, flexible tube with a miniature camera at the tip (colonoscope) into the rectum and guide it through the entire colon. The camera will send images of the colon to a nearby monitor, allowing the physician to examine the lining of the colon for polyps and signs of colon cancer. If abnormal tissue is found, the physician may perform a biopsy to obtain a sample for further analysis.

Often used for diagnostic purposes, colonoscopy is also widely considered to be the gold standard in colon cancer screening because it allows for the detection and removal of precancerous polyps before they can progress and develop into cancer.


Sigmoidoscopy is similar to colonoscopy but less extensive. A colonoscopy allows a physician to examine the entire colon, which is approximately 5-6 feet long and extends from the rectum to the cecum. A sigmoidoscopy, on the other hand, allows a physician to examine only the lower part of the colon, which includes the sigmoid colon and rectum.


A biopsy involves removing a small sample of tissue from the colon or rectum for microscopic examination by a pathologist, who can identify cancerous cells. During a colonoscopy or sigmoidoscopy, a physician may pass a special instrument through the scope to collect samples of abnormal tissue. Biopsy is considered to be the definitive method for diagnosing colon cancer.

Frequently asked questions (FAQs) about colon cancer diagnosis

The following FAQs-related articles provide additional information about colon cancer diagnosis:

Benefit from world-class care at Moffitt Cancer Center

If you would like to learn more about the diagnostic process for colon cancer, you can request an appointment with a specialist in Moffitt’s renowned Gastrointestinal Oncology Program by calling 1-888-663-3488 or submitting a new patient registration form online. We do not require referrals.

Helpful Links:

Reports Show Rise in Colorectal Cancer Among Young Adults