The main use for the FOBT is as a screen for early colon cancer. Blood in the stool may be the only symptom of early cancer. If the cancer is detected early, the chance that it will be curable is increased. The FOBT is not diagnostic for cancer; other follow-up procedures would need to be done to find the source of the bleeding because the blood may also indicate other gastrointestinal problems.

A secondary use of FOBT is to look for a cause of anemia, such as blood loss from a bleeding ulcer. If you have symptoms and signs of anemia, such as fatigue or a low hemoglobin and hematocrit, and/or unusually dark stools, your doctor may want to order the FOBT.

Fecal occult blood tests are most often done as part of a routine examination. The tests are used primarily as a screening tool for early detection of colon cancer. They are recommended to be done annually beginning at age 50 (by the American Cancer Society and other major organizations) or as directed by your doctor based on your family medical history. Most people who have them performed are asymptomatic.

Your doctor may also order an FOBT if he suspects that you have an unexplained anemia that might be caused by GI bleeding.

The FOBT test is normally negative. A positive test result will tell your doctor that you have abnormal bleeding occurring somewhere in your gastrointestinal tract. This blood loss could be due to ulcers, diverticulosis, bleeding polyps, inflammatory bowel disease, hemorrhoids, from swallowed blood due to bleeding gums or nosebleeds, or it could be due to benign or cancerous tumors. Anything that protrudes into the lumen (the empty space in the intestine), like a polyp or tumor, and is rubbed against by the fecal waste as it passes through has the potential to eventually bleed intermittently. Often, this small amount of blood is the first, and sometimes the only, sign of early colon cancer, making the FOBT a valuable screening tool.

Bleeding, especially from polyps and tumors, is intermittent, so blood is not uniformly distributed in all stool samples. Taking three different samples on three different days increases the chances of detecting bleeding that is intermittent.