The Fecal Occult Blood Test (FOBT) is a widely used, non-invasive screening tool designed to detect microscopic traces of blood in the stool, medically termed “occult” blood. This test is primarily utilized as a routine method to screen for early signs of colorectal cancer. A positive FOBT result indicates hidden blood was detected, signaling that further investigation is necessary to locate the source of the bleeding. The test cannot provide a diagnosis, but it flags the patient for a subsequent diagnostic evaluation of the digestive tract.
Understanding What the Result Means
A positive FOBT result should be viewed as an indication for action, not a definitive diagnosis of a serious disease. As a screening test, the FOBT identifies people who may benefit from diagnostic testing. The presence of occult blood confirms bleeding somewhere along the gastrointestinal tract, but it does not specify the location or the underlying cause.
This finding is not synonymous with cancer, as many non-cancerous conditions can cause blood to appear in the stool. The result shifts the patient from a screening pathway to a diagnostic pathway, requiring a more precise procedure to determine the specific source of the blood loss. This necessitates a thorough follow-up to pinpoint the exact etiology.
Common Reasons for a Positive Test
The detection of occult blood can be attributed to a variety of conditions, many of which are benign and easily treatable. Bleeding often originates from the lower digestive tract due to common structural issues like hemorrhoids, which are swollen veins in the rectum and anus, or anal fissures. Anal fissures are small tears in the lining of the anal canal often caused by passing hard stools.
Inflammatory conditions, such as colitis or inflammatory bowel disease (IBD) like Crohn’s disease and ulcerative colitis, can also cause intestinal bleeding. These diseases involve chronic inflammation and ulceration of the digestive tract lining. Diverticulosis, characterized by small, bulging pouches that form in the lining of the large intestine, can also cause bleeding when nearby blood vessels rupture.
A positive result may also be caused by precancerous growths called adenomatous polyps. While not cancerous themselves, these polyps can bleed and have the potential to develop into cancer over time. The FOBT is valuable because it detects bleeding from these polyps, allowing for their removal before they progress to malignancy.
Certain medications and dietary factors can interfere with the test, sometimes resulting in a false-positive finding. Nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin or ibuprofen, can irritate the stomach or intestines and cause minor bleeding. Additionally, for the older guaiac-based FOBT, consuming red meat or certain raw fruits and vegetables rich in peroxidase enzymes could lead to an inaccurate positive result.
Essential Follow-Up Diagnostic Steps
Following a positive FOBT, the standard next step is a diagnostic procedure called a colonoscopy. This procedure visualizes the entire inner lining of the large intestine to find the exact location and cause of the bleeding. A colonoscopy is necessary regardless of whether one or multiple stool samples tested positive for occult blood.
During a colonoscopy, the physician inspects the colon for polyps, inflammation, or other sources of bleeding. The procedure is both diagnostic and therapeutic, as the physician can immediately remove any polyps discovered, preventing potential cancer progression. Studies suggest that a positive FOBT is associated with approximately a 10% chance of cancer or a 37% chance of a polyp, highlighting the importance of this follow-up.
Timely completion of the colonoscopy is recommended, ideally within 60 days of the positive screening result. Delays in this diagnostic step are associated with an increased risk of finding colorectal cancer at a more advanced stage. If the colonoscopy does not reveal the source of the bleeding, or if the source is suspected to be higher up in the digestive tract, supplementary tests like an upper endoscopy or CT colonography may be considered.