Fecal Occult Blood Test Definition

Fecal Occult Blood Test (FOBT) – a laboratory test to determine whether there is microscopic (occult) blood in the stool, primarily to screen for colorectal cancer although other conditions, such as inflammatory bowel disease (ibd) and diverticulosis, can also cause occult bleeding.

Two kinds of FOBT kits are available for self-sampling at home, one that a laboratory tests and the other that shows immediate results.

For the conventional guaiac test, the person receives a kit from the doctor. The kit contains three cards onto which the person applies a small stool sample, one sample each day for three days. The cards go into a prepaid envelope for mailing to the laboratory (or may be returned to the doctor’s office).

The lab applies a chemical, guaiac, that reacts with heme, a component of the hemoglobin in blood. The reaction produces a blue coloration, a positive result. No color change indicates a negative result (no blood is present).

Any positive fecal occult blood test (FOBT) result requires further medical evaluation to determine the source of the bleeding and to rule out serious conditions such as colorectal cancer.

Tests that show immediate results are available in most pharmacies and drugstores without a doctor’s prescription. They contain reagent tissues that the person drops into the toilet following a bowel movement (before flushing). The tissue turns blue-green if there is any heme present, indicating blood and remains colorless when no blood is present. As with the conventional test, the person tests three bowel movements over three days. The kit includes a card that the person can fill out and send to his or her doctor or keep for personal health records.

The FOBT is a good test for colorectal cancer because the intestinal polyps that are its starting points bleed easily, though the bleeding often is not apparent with visual examination of the stool. Many health conditions can cause positive results, such as ulcers, diverticular diseasehemorrhoids, and anal fissure.

Certain foods and other substances can cause false-positive or false-negative results with guaiac-based tests; test instructions may advise avoiding them for 48 hours before performing the tests (seven days for aspirin and nonsteroidal anti-inflammatory drugs [nsaids]). Women should do FOBT when they are not menstruating.

Substances that may Alter FOBT Results

Substances that may Alter FOBT Results
False-Positive ResultsFalse-Negative Results
red meatvitamin C supplements
cruciferous vegetablescitrus fruits and juices

Health-care professionals recommend FBOT annually starting at age 45 for both men and women as a screening for colorectal cancer.

Doctors also may request FBOT when they suspect conditions that can cause gastrointestinal bleeding and when there is anemia for no apparent reason.


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