The fecal occult blood test (FOBT), also known as the stool occult test or hemoccult test, is a noninvasive test that screens stool samples for the presence of blood in the stool that is not visible to the naked eye.
- Negative result: FOBT is considered negative if blood is not detected in the stool sample. If a person is at risk of colon cancer, the test may need to be repeated every year.
- Positive result: FOBT is considered positive if blood is detected in the stool sample. An additional test such as a colonoscopy may be needed to locate the source and cause of the bleeding.
FOBT is most commonly used as a screening test for colorectal cancer. Blood in the stool indicates the likelihood of bleeding in the digestive tract, which can be a sign of several conditions such as polyps, tumors, or cancer in the colon or rectum.
In addition to colon cancer screening, the most common indications for FOBT include anemia and gastrointestinal bleeding. Moreover, it can be used to help differentiate irritable bowel syndrome and inflammatory bowel disease.
FOBT can be performed both inpatient and outpatient. In an inpatient setting, the stool is obtained during a digital rectal examination. In an outpatient setting, the patient collects their stool sample at home and submits it to a lab for further investigation.
What are different types of fecal occult blood tests?
- Guaiac fecal occult blood test (gFOBT): To ensure accuracy, the patient will need to avoid certain foods and medications beforehand. Stool samples are collected from 3 separate bowel movements and each is stored in a clean container, labeled, and sent to a lab for testing. In the lab, the stool sample is smeared on a test card coated with guaiac, which is a plant-based substance. The presence of occult blood is confirmed by a color change of the card.
- Fecal immunochemical test or immunochemical fecal occult blood test or immunochemical FOBT: This test uses a specialized protein called an antibody that attaches to the hemoglobin, which is the oxygen-carrying part of red blood cells. The stool sample is placed in a tube or on a card and then sent to a lab for testing.
The immunochemical test is preferred over gFOBT because it doesn't require any diet restrictions and can be done any time. Additionally, it is more sensitive in detecting cancer and has largely replaced gFOBT for colorectal screening.
Which conditions cause a positive fecal occult blood test?
A positive fecal occult blood test may be caused by various conditions of the gastrointestinal system other than cancer, including:
- Anal fissure: Small tear caused by excessive stretching of the anal tissue while passing hard stools, which results in rectal bleeding and causes severe pain.
- Crohn's disease: A type of chronic inflammatory bowel disease that causes inflammation of the digestive tract, leading to abdominal pain, severe diarrhea, fatigue, weight loss, and rectal bleeding.
- Diverticular disease: Condition in which small pockets called diverticula form and push outward through weak spots in the wall of the colon, caused by straining during bowel movements. When the pockets get infected or inflamed, it leads to a condition called diverticulitis, which causes nausea, vomiting, and rectal bleeding.
- Colonic polyps: Growths that appear on the inner surface of the large intestine and commonly cause rectal bleeding. Polyps vary in size and number:
- Hyperplastic polyps: Usually harmless
- Adenomatous polyps: Most common and have the potential to develop into colon cancer
- Malignant polyps: Polyps with cancer cells
- Hemorrhoids: Also known as piles and one of the common causes of rectal bleeding. They are caused by swollen veins in the lowest part of the rectum and anus.
- Intestinal infections: Caused by salmonella, enteroinvasive and enterohemorrhagic Escherichia coli, shigella, neisseria, yersinia, tuberculosis, campylobacter, and strongyloides.
- Meckel’s diverticulum: Congenital anomaly that presents as a small bulge in the gastrointestinal tract. Inflammation of the diverticula due to blocked feces can result in fever, diarrhea, and bloody stools.
- Ulcerative colitis: Inflammatory condition that causes irritation and open sores in the colon. Inflammation usually begins in the rectum and spreads upward. Symptoms worsen over time, and patients experience diarrhea, weight loss, abdominal cramping, anemia, and blood or pus during bowel movements.
- Ulcers: Sores on the lining of the stomach, small intestine, or esophagus. They may cause severe bleeding.
- Vascular conditions (problems in the blood vessels): Examples include angiodysplasia, venous ectasia, variceal bleeding, hemangioma, and gastric antral vascular ectasia.
Who may need a fecal occult blood test?
Those with risk factors of colon cancer may need regular fecal occult blood test screening:
What are the limitations of fecal occult blood tests?
Limitations of fecal occult blood tests include the following:
- Not always accurate: FOBT may show a false negative result when cancer or polyps are present but do not bleed. It may show a false positive result when there is no cancer and if bleeding occurs due a stomach ulcer, hemorrhoids, or blood ingested from the mouth or nose.
- May require additional testing: If the FOBT result is positive, further testing may be needed such as a colonoscopy to examine the inside of the colon.
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