colon cancer symptom : Occult Blood Tests

Bleeding in the digestive system can be caused by something as insignificant as a little irritation or as serious as cancer. When bleeding is profuse, a person can vomit blood (hematemesis), pass bright red blood in the stool (hematochezia), or pass black, tarry stool (melena). Amounts of blood too small to be seen or to change the appearance of stool can be detected chemically, and the detection of such small amounts may provide early clues to the presence of ulcers, cancers, and other abnormalities.

During a rectal examination, the doctor obtains a small amount of stool on a gloved finger. This sample is placed on a piece of filter paper impregnated with a chemical (guaiac). After another chemical is added, the color of the sample will change if blood is present. Alternatively, the person can take home a kit containing the impregnated filter papers. The person places samples of stool from about three different bowel movements on the filter papers, which are then mailed in special containers back to the doctor for testing. If blood is detected, further examinations are needed to determine the source.

http://www.merck.com/mmhe/sec09/ch119/ch119c.html#sec09-ch119-ch119c-141