The Occult Blood Test (OBT) is a non-invasive screening tool designed to detect microscopic amounts of blood in a stool sample. The term “occult” means hidden, signifying that the blood is not visible to the naked eye. This test is generally performed at home and serves as an initial check for bleeding within the gastrointestinal (GI) tract. The presence of minute traces of blood can signal an underlying health issue that requires further investigation.
Why Doctors Use Occult Blood Testing
Doctors utilize the Occult Blood Test as a primary method for population-based screening, particularly for the early detection of colorectal cancer. This type of cancer often causes slow, intermittent bleeding from precancerous polyps or tumors before any physical symptoms emerge. By identifying this hidden bleeding, the test can flag individuals who require more detailed follow-up procedures.
The term Fecal Occult Blood Test (FOBT) is often used interchangeably with OBT. Modern practices increasingly rely on the Fecal Immunochemical Test (FIT). The FIT specifically uses antibodies to detect human hemoglobin, the protein in red blood cells, making it more specific to lower GI bleeding than older methods.
Beyond cancer screening, the OBT helps investigate unexplained iron-deficiency anemia. Gastrointestinal bleeding can lead to a continuous loss of iron over time, resulting in anemia. Finding occult blood can direct the diagnostic search toward the digestive system as the source of iron loss. The test is also employed to help distinguish between conditions like Irritable Bowel Syndrome (IBS), which typically does not cause bleeding, and Inflammatory Bowel Disease (IBD), such as ulcerative colitis, where bleeding is a common feature.
Collecting the Sample
Collecting the sample for an OBT is a straightforward process typically performed by the patient in their home. The physician provides a specialized kit, which includes instructions and the necessary tools for collection and return. For the modern Fecal Immunochemical Test (FIT), the process is simple and does not require any special dietary or medication changes beforehand.
The patient first uses a collection device to obtain a stool sample without it touching the toilet water. A small applicator stick is then used to scrape a sample and smear it onto a designated area on the collection card or insert it into a tube. Since bleeding from polyps or lesions can be intermittent, many kits require samples from one to three separate bowel movements to increase the chance of detection.
For older guaiac-based tests (gFOBT), which detect the peroxidase activity of blood, patients were required to avoid red meat, certain raw fruits, and vegetables, as well as some medications like Vitamin C and nonsteroidal anti-inflammatory drugs (NSAIDs) for several days. These restrictions were necessary to prevent false results, a complication largely avoided by the modern FIT. Once collected, the sealed sample is returned to the laboratory for analysis.
Interpreting Test Outcomes
The outcome of an Occult Blood Test is categorized as either negative or positive for the presence of blood in the stool. A negative result means that no hidden blood was detected. However, a negative result does not rule out polyps or cancer, as these lesions may not bleed consistently, leading to a false-negative result. Individuals with an average risk are typically advised to repeat the screening test on a yearly basis.
A positive OBT result indicates that microscopic blood was found in the stool sample. A positive test is not a diagnosis of cancer but rather an indication of bleeding somewhere in the gastrointestinal tract. This result prompts the need for a follow-up, more invasive procedure, most commonly a colonoscopy, to visually inspect the colon and determine the precise source of the bleeding.
The bleeding can stem from various non-cancerous conditions, including ulcers, hemorrhoids, diverticulosis, or inflammatory bowel disease. These conditions are common causes of false-positive results. Taking certain medications or having conditions that cause upper GI bleeding can also lead to a positive result with the older guaiac-based test, though the FIT is less prone to this. A physician’s consultation is necessary to interpret the outcome and plan the appropriate diagnostic path forward.