Colorectal cancer affects the large intestine (colon) and the rectum. It is the third most common type of cancer in the United States and the second leading cause of deaths from cancer. 

Screening and diagnostic tests for colon cancer may include testing for blood in the stool (fecal occult blood testing, or FOBT), a digital rectal exam, flexible sigmoidoscopy, a barium enema, or colonoscopy. 

Checking for hidden (occult) blood in the stool is a simple colon cancer screening test people can do at home. Testing kits are available at pharmacies without a prescription, or your doctor may order a testing kit for you to use at home.

The most common test is the stool guaiac test (such as Hemoccult II, Hemoccult II Sensa, HemeSelector, or Hemoquant). A small sample of stool is applied to a chemically treated card, then a chemical developer solution is added. If the card turns blue, there is blood in the stool.

Fecal Occult Blood Test (FOBT)
Results

The way results are displayed may vary, depending on the type of test you are using. If you are given stool guaiac cards by your doctor, you may not be able read the results yourself. Instead, you will return the cards to your doctor, who will then send them to a lab to be developed.

Normal

A normal test (no color change) indicates that at the time you collected your stool samples there was no detectable blood in your stool. Normal test results are called negative.

A negative test result does not rule out the possibility of colon cancer or colon polyps. FOBT is positive in only 30% to 50% of the people who have colon cancer. Therefore, it is important that you continue to have an FOBT once a year.

Abnormal

An abnormal result (blue color change) may indicate that at the time of the test there was detectable blood in the stool. Abnormal test results are called positive.

If you have a positive test result, it may be caused by something other than colon polyps or colon cancer.

• If blood in the stool may be caused by red meat you have eaten, menstrual bleeding, hemorrhoids, Crohn's disease, ulcerative colitis, or a stomach ulcer and you do not have risk factors for colon polyps or colon cancer, your doctor may want you to repeat the FOBT. If the repeat test is negative, you may resume regular annual FOBT screening. If the repeat test is positive, you may have further testing, such as colonoscopy, barium enema, or flexible sigmoidoscopy. 
• If blood in the stool may be caused by red meat you have eaten, menstrual bleeding, hemorrhoids, Crohn's disease, ulcerative colitis, or a stomach ulcer and you have risk factors for colon polyps or colon cancer, your doctor may move ahead with further testing, such as colonoscopy, barium enema, or flexible sigmoidoscopy, without doing another FOBT.

If you have a positive test result, it may be caused by a polyp, a precancerous polyp, or cancer. With a positive result, there is about a 7% to 14% probability that you have early-stage colon cancer. About 30% of the time a positive result is caused by a large polyp. About 50% of the time there is no abnormality found that can explain the positive FOBT result.

• If you have a positive test result, you are younger than 50, and you do not have risk factors for colon polyps or colon cancer, your doctor may want you to repeat the FOBT. If the repeat test is negative, you may resume regular annual FOBT screening. If the repeat test is positive, you may have further testing, such as colonoscopy, barium enema, or flexible sigmoidoscopy. 
• If you have a positive test result, you are older than 50, and you have not been evaluated recently for colon polyps, colon cancer, Crohn's disease, ulcerative colitis, or stomach ulcers, you will probably need further testing, such as colonoscopy, barium enema, or flexible sigmoidoscopy.
• If you have a positive test result and you have been evaluated recently for colon polyps, colon cancer, Crohn's disease, ulcerative colitis, or stomach ulcers, your doctor may simply have you repeat the FOBT or you may resume regular annual FOBT screening.