Occult gastrointestinal bleeding refers to the presence of blood in the stool that is not visible to the naked eye. This microscopic blood loss most often originates from somewhere within the extensive gastrointestinal (GI) tract. Occult bleeding is usually detected during routine screening tests, such as those for colorectal cancer, or when a patient is being investigated for unexplained iron-deficiency anemia. The continuous, small volume of blood loss slowly depletes the body’s iron stores, which is often the first clinical sign of a problem. This hidden bleeding requires specific testing to be confirmed, leading to a focused search for the source within the digestive system.
Screening Methods for Hidden Blood
The primary way to find hidden blood in the stool involves non-invasive methods designed to detect trace amounts of blood components. The two main tests are the guaiac-based Fecal Occult Blood Test (gFOBT) and the Fecal Immunochemical Test (FIT). Both tests are easy to perform at home and serve as an initial screen to determine if further diagnostic investigation is needed.
The gFOBT works by detecting the peroxidase activity of the heme component in blood, which is the iron-containing part of hemoglobin. This chemical reaction can be triggered by non-human sources, meaning that consuming red meat or certain raw vegetables like broccoli can lead to a false-positive result. Medications such as aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs) can also cause bleeding that triggers a positive result, complicating interpretation.
The Fecal Immunochemical Test (FIT) offers a more specific approach, relying on antibodies to detect the human globin protein found in hemoglobin. Because the antibodies are specific to human blood, the FIT does not require dietary restrictions. FIT is generally preferred today because it is more sensitive and specific for lower GI bleeding, as the globin protein degrades as it travels through the upper GI tract.
Common Causes of Occult Gastrointestinal Bleeding
The conditions that cause occult bleeding typically share a mechanism of slow, persistent damage to the lining of the digestive tract. Colorectal polyps are a frequent source, often bleeding due to microerosion or a thin, fragile surface epithelium. As stool passes, friction can cause minor injury to the polyp’s surface, resulting in minute, invisible blood loss.
Colorectal cancer also causes hidden blood loss through surface erosion and ulceration within the tumor mass. The tumor’s fragile blood vessels are easily damaged, and the bleeding is often slow because the body’s clotting mechanisms attempt to seal the site.
Peptic ulcers, which are open sores in the lining of the stomach or the first part of the small intestine, are another common cause. These ulcers erode the protective mucosal layer, exposing underlying blood vessels to digestive acids and enzymes. If the erosion is shallow, it leads to a slow, chronic ooze of blood detectable only through occult blood tests.
Inflammatory Bowel Disease (IBD), including Crohn’s disease and ulcerative colitis, involves chronic inflammation that causes extensive shallow ulcerations and erosions along the GI tract lining. This widespread inflammation makes the tissue fragile and prone to bleeding from minimal trauma as intestinal contents pass through.
Non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen and aspirin, can also induce occult bleeding by interfering with the protective mechanisms of the GI lining. NSAIDs inhibit cyclooxygenase enzymes, which are necessary for producing mucus and maintaining blood flow to the lining. This inhibition leads to erosions and shallow ulceration in the stomach, small intestine, and colon.
Necessary Steps After a Positive Test
A positive occult blood test result is not a diagnosis but indicates that a source of bleeding exists and must be found through diagnostic procedures. The standard next step is a colonoscopy, which allows a physician to visually inspect the entire large intestine. This procedure identifies the exact location of the bleeding, confirms the presence of polyps or cancer, and often allows for immediate treatment, such as removing polyps.
Guidelines recommend that this follow-up colonoscopy should be performed promptly, ideally within 60 days of the positive screening test, to minimize disease progression. If the colonoscopy is negative, but symptoms like iron-deficiency anemia persist, a search for an upper GI source is initiated using an esophagogastroduodenoscopy (EGD). This procedure uses an endoscope to visualize the esophagus, stomach, and the initial part of the small intestine.
If both the colonoscopy and EGD are negative, attention shifts to the small bowel, which is inaccessible to standard endoscopes. In these cases, capsule endoscopy is often employed, where the patient swallows a pill-sized camera that transmits images of the small intestine. Identifying the precise source of occult bleeding is necessary, as failure to investigate can delay diagnosis.