Blood in the feces that is not visibly apparent is referred to as fecal occult blood (FOB). A fecal occult blood test (FOBT) checks for hidden (occult) blood in the stool (feces). Newer tests look for globin, DNA, or other blood factors including transferrin, while conventional stool tests look for heme.
The purpose of fecal occult blood testing (FOBT) is to detect subtle blood loss in the gastrointestinal tract, anywhere from the mouth to the colon. Positive tests ("positive stool") may result from either upper gastrointestinal bleeding or lower gastrointestinal bleeding and warrant further investigation for peptic ulcers or a malignancy (such as colorectal cancer or gastric cancer). The test does not directly detect colon cancer, but is often used in clinical screening for that disease. It can also be used to look for active occult blood loss in anemia or when there are gastrointestinal symptoms.
The different methods of testing for fecal occult blood actually test for particular components of blood or for aberrantly-expressed cellular markers from the intestinal mucosa. The four methods look at different properties such as antibodies, heme, globin, or porphyrins in blood, or at DNA from cellular material such as from lesions of the intestinal mucosa.
In healthy people, about 0.5 to 1.5 ml of blood escapes blood vessels into the stool each day. Significant amounts of blood can be lost without producing visible blood in the stool (e.g., 200 ml in the stomach, 100 ml in the duodenum, and lesser amounts in the lower intestine). Tests for occult blood identify lesser blood loss.
The stool guaiac test for hidden (occult) blood in the stool can be done at home or in the doctor's office, or can be performed on samples submitted to a clinical laboratory (Figure 1). Testing kits are available at pharmacies in some countries without a prescription, or a health professional may order a testing kit for use at home (Figure 2). If a home fecal occult blood test detects blood in the stool it is recommended to see a health professional to arrange further testing. An estimated 1–5% of large tested populations have a positive fecal occult blood test. Of those, about 2–10% have cancer, while 20–30% have adenomas.
Gastrointestinal bleeding has many potential sources, and positive test results usually lead to further testing for the bleeding site, usually looking at possible lower gastrointestinal bleeding before looking at upper gastrointestinal bleeding causes, unless there are other clinical clues. The common causes are cancer (colorectal cancer, gastric cancer, 2-10%), adenoma or polyps (20-30%), bleeding peptic ulcer, angiodysplasia of the colon, and sickle cell anemia.
In the event of a positive fecal occult blood test, the next step in the workup is a form of visualization of the gastrointestinal tract by one of several means: sigmoidoscopy, an examination of the rectum and lower colon with a lighted instrument to look for abnormalities, such as polyps; colonoscopy, a more thorough examination of the rectum and entire colon; virtual colonoscopy; endoscopy refers to upper gastrointestinal endoscopy sometimes performed with chromoendoscopy; and double contrast barium enema: a series of X-rays of the colon and rectum.