An abnormal fecal occult blood test result indicates the presence of “occult blood,” which is blood in the stool not visible to the naked eye. This microscopic bleeding in the digestive tract is detected only through chemical testing. While a positive result can cause anxiety, this finding is not a diagnosis but a signal requiring further investigation. This screening result prompts a search for the source of the bleeding, which can range from minor irritation to more serious underlying health conditions.
What the Fecal Occult Blood Test Measures
The fecal occult blood test (FOBT) is a non-invasive screening tool designed to detect trace amounts of hemoglobin, the protein found in red blood cells. There are two primary types of these tests, operating on different principles to detect hidden blood. The traditional Guaiac-based FOBT (gFOBT) uses a chemical reaction to detect the peroxidase activity of heme, a component of the blood protein. This method detects bleeding from anywhere in the gastrointestinal tract but is prone to interference from certain foods and medications.
A more modern approach is the Fecal Immunochemical Test (FIT), which uses antibodies to detect the human globin portion of hemoglobin. Because globin breaks down rapidly, a positive FIT result is highly specific for bleeding in the lower gastrointestinal tract, such as the colon or rectum. Neither test identifies the location or severity of the bleeding; they simply confirm blood is present in the stool sample. A positive result serves as a trigger for a more definitive diagnostic procedure.
Everyday Causes of a Positive Result
An abnormal test result does not automatically point to a severe disease; it can frequently be traced back to common, benign sources of bleeding. Hemorrhoids are the most frequent cause of occult blood, representing swollen veins in the anus or lower rectum irritated by the passage of hard stool. Similarly, anal fissures, small tears in the lining of the anal opening, can cause minor bleeding detected by the test. These conditions are localized and usually respond well to conservative treatments.
Certain medications cause mild gastrointestinal irritation and bleeding, potentially leading to a positive screening result. Nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin, ibuprofen, and naproxen can damage the protective lining of the stomach and small intestine, resulting in low-level blood loss. For the older gFOBT, the test’s reliance on a chemical reaction means dietary factors can lead to a false-positive reading. Eating rare red meat or certain raw vegetables, such as broccoli or radishes, can mimic blood presence due to their own peroxidase content.
To minimize false results, specific preparation protocols are required before taking a gFOBT, including avoiding these foods and medications for several days. Minor inflammation in the stomach (gastritis) or a small peptic ulcer can also cause sufficient bleeding to trigger a positive result. These causes are usually self-limiting or easily manageable, but still require a full medical evaluation to ensure no serious condition is missed.
Underlying Conditions Needing Further Investigation
While many positive results have benign explanations, the primary reason for screening is to detect conditions requiring prompt medical attention. The most significant finding a positive test might indicate is the presence of pre-cancerous growths called adenomatous polyps. These abnormal growths on the lining of the colon or rectum may bleed intermittently, and their removal prevents potential development into colorectal cancer.
Colorectal cancer is a serious cause of occult bleeding, and the test’s value lies in detecting the cancer at an early, highly treatable stage before symptoms become obvious. Early detection significantly increases the chance of a cure. A positive result can also point toward chronic inflammatory conditions of the bowel, known as Inflammatory Bowel Disease (IBD). This includes Ulcerative Colitis and Crohn’s disease, which cause inflammation, ulcers, and bleeding along the digestive tract.
Bleeding may also originate from diverticular disease, where small pockets (diverticula) form in the wall of the colon and can sometimes bleed or become inflamed. A positive gFOBT could also indicate bleeding higher up in the digestive system, such as a peptic ulcer in the stomach or small intestine, or a tumor in the upper GI tract. These conditions necessitate comprehensive diagnostic investigation to pinpoint the exact location and nature of the bleeding source.
Necessary Follow-Up Diagnostic Procedures
Following an abnormal fecal occult blood test, the most important next step is a diagnostic procedure to visually inspect the digestive tract. The definitive follow-up test is a colonoscopy, which allows a physician to examine the entire lining of the colon and rectum. During this procedure, the doctor can identify the precise source of the bleeding, take tissue samples for biopsy, and remove any polyps found, preventing potential cancer development.
It is recommended that this colonoscopy be performed promptly, ideally within 60 days of the positive screening result, to minimize the risk of disease progression. Delaying this follow-up is inappropriate, even if the patient suspects the bleeding is due to a known benign cause like hemorrhoids or medication use. If the colonoscopy is negative but upper GI bleeding is suspected, the physician may recommend an upper endoscopy. This procedure examines the esophagus, stomach, and the beginning of the small intestine to check for ulcers or other issues.