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Stool Occult Blood Tests

By Walter W. Chan, MD, MPH, Assistant Professor of Medicine;Director, Center for Gastrointestinal Motility, Division of Gastroenterology, Hepatology, and Endoscopy, Harvard Medical School;Brigham and Women's Hospital

Bleeding in the digestive system can be caused by something as insignificant as a little irritation or as serious as cancer. Amounts of blood that are too small to be seen (called occult blood) or to change the appearance of stool can be detected with chemicals. The detection of such small amounts of blood may provide an early clue that ulcers, cancers, and other abnormalities are present.

During a rectal examination, the doctor may obtain a small amount of stool on a gloved finger. This sample is placed on a piece of filter paper infused with a chemical (guaiac). After another chemical is added, the sample will change color if blood is present. More preferably, the person can take home a kit containing the 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.