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Stool Occult Blood Tests


Jonathan Gotfried

, MD, Lewis Katz School of Medicine at Temple University

Full review/revision Apr 2021 | Modified Sep 2022

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 in the stool 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. Stool tests for 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.

Fecal immunochemical test (FIT)

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.

These newer immunochemical tests are more accurate than older guaiac-based stool tests and are preferred by most medical society guidelines for colorectal cancer screening Screening tests Family history and some dietary factors (low fiber, high fat) increase a person’s risk of colorectal cancer. Typical symptoms include bleeding during a bowel movement, fatigue, and weakness... read more Screening tests . The fecal immunochemical 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 than the fecal immunological test alone and is done every 3 years. However, the FIT-DNA test is very expensive.

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