With proper screening, colon cancer should be detected BEFORE the development of symptoms, when it is most curable.

Most cases of colon cancer have no symptoms. The following symptoms, however, may indicate colon cancer:

• Diarrhea, constipation, or other change in bowel habits that does not resolve
• Blood in the stool
• Unexplained anemia (anemia in any adults other than menstruating women should almost always be evaluated by a colonoscopy)
• Abdominal pain and tenderness in the lower abdomen
• Intestinal obstruction
• Weight loss with no known reason
• Stools narrower than usual

A physical examination rarely shows any abnormalities, although an abdominal mass may be present. A rectal examination may reveal a mass in patients with rectal cancer, but not colon cancer.

A colonoscopy or sigmoidoscopy may reveal evidence of cancer. However, only colonoscopy (NOT sigmoidoscopy) examines the entire colon.

A fecal occult blood test (FOBT) may detect small amounts of blood in the stool, a possible indicator of colon cancer. However, this test is often negative in patients with colon cancer. Not all polyps bleed, and not all polyps bleed all the time. That is why a FOBT must be used with one of the other more invasive screening measures (e.g., colonoscopy or sigmoidoscopy). Finally, a positive FOBT doesn't necessarily mean the person has cancer -- "false positives" can be caused by some medications and other factors.

A blood count may reveal evidence of anemia with low iron levels. A CT scan may show an abdominal mass, although this test is not very good at detecting colon cancer.