When colon cancer is suspected, either a lower GI series (barium enema x-ray) or colonoscopy is performed to confirm the diagnosis and to localize the tumor.

A barium enema involves taking x-rays of the colon and the rectum after the patient is given an enema with a white, chalky liquid containing barium. The barium outlines the large intestines on the x-rays. Tumors and other abnormalities appear as dark shadows on the x-rays. For more information, please read the Lower Gastrointestinal Series (Barium Enema) article.

Colonoscopy is a procedure whereby a doctor inserts a long, flexible viewing tube into the rectum for the purpose of inspecting the inside of the entire colon. Colonoscopy is generally considered more accurate than barium enema x-rays, especially in detecting small polyps. If colon polyps are found, they are usually removed through the colonoscope and sent to the pathologist.

The pathologist examines the polyps under the microscope to check for cancer. While the majority of the polyps removed through the colonoscopes are benign, many are precancerous. Removal of precancerous polyps prevents the future development of colon cancer from these polyps. For more information, please read the Colonoscopy article.

If cancerous growths are found during colonoscopy, small tissue samples (biopsies) can be obtained and examined under the microscope to confirm the diagnosis. If colon cancer is confirmed by a biopsy, staging examinations are performed to determine whether the cancer has already spread to other organs. Since colorectal cancer tends to spread to the lungs and the liver, staging tests usually include chest x-rays, ultrasonography, or a CAT scan of the lungs, liver, and abdomen.

Sometimes, the doctor may obtain a blood test for CEA (carcinoembyonic antigen). CEA is a substance produced by some cancer cells. It is sometimes found in high levels in patients with colorectal cancer, especially when the disease has spread.