A fecal occult blood test (FOBT) is used to check for bleeding somewhere in the digestive tract, especially the intestines. This type of bleeding can be linked to conditions like ulcers, colitis, Crohn’s disease, intestinal polyps, or colorectal cancer.
Occult blood in the stool means there is blood mixed in with the stool that you cannot see with the naked eye. It is only detected through lab testing.
The fecal occult blood test is generally simple to do, but preparation matters. It is commonly done by collecting stool samples over three days and avoiding certain foods and medicines that can affect results.
Causes of occult blood in the stool
The main causes of occult blood in the stool include:
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Benign intestinal polyps;
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Hemorrhoids;
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Stomach or duodenal ulcers;
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Ulcerative colitis;
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Crohn’s disease;
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Colorectal cancer.
To identify the correct cause of blood in the stool, it is common for a clinician to order a colonoscopy or an upper endoscopy after the fecal occult blood test, especially when bleeding is not due to hemorrhoids.
Both tests use a thin tube with a small camera at its tip, allowing the doctor to examine the inside of the stomach and intestines for possible lesions and to help make a diagnosis.
Test prep
To do a fecal occult blood test, it is important to follow your provider's recommendations during the stool collection period, which is usually three days, because certain factors can interfere with the results.
To prepare for the test, it is recommended to:
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Avoid foods such as radishes, cauliflower, broccoli, beets, beans, peas, lentils, chickpeas, corn, olives, peanuts, spinach, or apples;
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Avoid medicines that can irritate the stomach lining, such as anti-inflammatory drugs or aspirin, because they may cause bleeding and lead to a false-positive result, and also avoid vitamin C and iron supplements;
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Do not perform the test within three days after your menstrual period;
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Do not perform the fecal occult blood test if there is bleeding from the gums or nose, since swallowed blood can pass into the stool.
If stool collection happens under any of these conditions, inform the lab so this can be considered during interpretation. However, in many cases, the test may need to be repeated to confirm the result
Interpreting results
Possible results of the fecal occult blood test include:
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Negative: no occult blood is detected in the stool, suggesting a low risk of gastrointestinal changes;
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Positive: occult blood has been detected in the stool. The doctor will usually recommend further testing (most often a colonoscopy) to determine the source of bleeding and guide appropriate treatment.
If results are positive, or if results are negative but there are still some concerning findings, a doctor may request a repeat test to confirm the result or recommend a colonoscopy based on the person’s clinical history.
False-positive results are results in which blood is detected by the test but it does not reflect the patient’s true condition, and according to the National Cancer Institute, stool-based tests like FOBT can sometimes appear abnormal even when no cancer is found.
In some cases, even with a negative result, a doctor may still request a colonoscopy if the patient is at high risk of developing colon cancer to confirm there are no abnormalities. Although uncommon, colon cancer can exist without bleeding.