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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. Chemicals can be used to detect small amounts of blood that are too small to be seen or change the appearance of stool (called occult blood). The detection of such small amounts of blood may provide an early clue that ulcers, cancers, or other abnormalities are present. Genetic material from cancer cells may also be used to detect cancer.

Guaiac-based stool test

For this test, a chemical called guaiac is used to detect blood in the stool. A doctor may obtain a stool sample for this test during a rectal examination by using a gloved finger. This sample is placed on a piece of filter paper infused with guaiac. A second liquid chemical (peroxidase) is added and the sample changes 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 back to the doctor for testing.

If blood is detected, further tests are needed to determine the source.

Before doing this test, people may be told to avoid certain foods (such as red meat) and to limit their intake of vitamin C to less than 250 milligrams per day for 3 days before.

Immunochemical stool test

The fecal immunochemical test (FIT) uses antibodies directed against human hemoglobin (a protein in red blood cells that gives blood its red color) to detect blood in the stool. For this test, people collect samples by using a kit at home similar to the guaiac-based stool test.

If blood is detected, further tests are needed to determine the source.

This test requires no dietary, drug, or vitamin restrictions.

Newer immunochemical tests are more accurate than older guaiac-based stool tests and are preferred by some medical society guidelines for colorectal cancer screening. The FIT test is recommended annually for screening for colorectal cancer.

Genetic stool test (FIT-DNA test)

This approach uses a combination of a test for genetic material (DNA) associated with colorectal cancer and the fecal immunological test (FIT—antibodies directed against human hemoglobin are used to detect blood in the stool). People collect samples for this test by using a kit at home and mailing the samples to the laboratory.

If this test is abnormal, further tests are needed to determine the source.

This test requires no dietary, drug, or vitamin restrictions.

This test is more accurate and is done less frequently than FIT alone. However, the FIT-DNA test is very expensive.