The Occult Blood Test (OBT), often called the Fecal Occult Blood Test (FOBT), is a non-invasive screening tool. It detects minute amounts of blood in a stool sample that are not visible to the naked eye. The primary purpose of this laboratory analysis is to find hidden bleeding within the gastrointestinal tract, which signals the need for further diagnostic investigation.
Defining Occult in Medical Context
The term “occult” in medicine describes a symptom or condition that is not readily apparent during a routine physical examination. Occult blood refers to microscopic traces of blood in the stool, present in amounts too small to change the color or appearance of the feces. This contrasts with “gross” bleeding, which involves visible blood, such as bright red blood or black, tarry stools (melena).
Occult blood often results from slow, intermittent bleeding anywhere along the digestive pathway. Because the blood is not in macroscopic amounts, specialized chemical or immunologic tests are required to confirm its presence. This hidden bleeding is significant because chronic blood loss can lead to indirect symptoms like unexplained fatigue or anemia.
Clinical Applications of the Test
The most common application of the Fecal Occult Blood Test is for colorectal cancer screening in average-risk individuals who are not experiencing symptoms. Colorectal cancer often grows slowly, and early-stage tumors or polyps may bleed intermittently before causing noticeable symptoms. OBT screening identifies this early bleeding, allowing for timely intervention that improves treatment outcomes.
Screening guidelines recommend that average-risk individuals begin annual or biennial testing starting at age 45 or 50. Beyond cancer screening, the OBT is also utilized to investigate unexplained iron-deficiency anemia, which signals chronic, low-grade blood loss in the digestive tract. The test also helps differentiate between gastrointestinal disorders, such as inflammatory bowel diseases, which cause bleeding, and conditions like irritable bowel syndrome, which typically do not.
Understanding Different Testing Methods
Two primary types of Fecal Occult Blood Tests are currently in use, each employing a different mechanism to detect the hidden blood.
Guaiac-based Fecal Occult Blood Test (gFOBT)
The Guaiac-based Fecal Occult Blood Test (gFOBT) is the older method, relying on a chemical reaction to identify the blood component heme. The test card contains guaiac, which changes color to blue upon contact with the peroxidase enzyme found in hemoglobin. Because this chemical approach can react to peroxidases in certain foods, strict dietary restrictions are necessary before sample collection.
Patients must avoid red meat, which contains animal heme, for several days prior to testing to prevent false-positive results. Additionally, high doses of Vitamin C and certain nonsteroidal anti-inflammatory drugs (NSAIDs) may interfere with the chemical reaction, potentially leading to false-negative results.
Fecal Immunochemical Test (FIT)
The Fecal Immunochemical Test (FIT) is the modern and preferred method. This test utilizes antibodies highly specific to human hemoglobin, targeting the globin protein component. Since the antibodies only bind to human blood, the FIT is not affected by dietary animal blood or other food peroxidases, eliminating the need for dietary restrictions.
Because the globin protein degrades in the upper digestive tract, the FIT is highly selective for bleeding originating in the lower GI tract, such as the colon or rectum. This makes it a more targeted screening tool for colorectal cancer than the gFOBT. The FIT’s greater specificity and ease of use have made it the dominant technology in many screening programs.
Interpreting Results and Follow-Up Actions
An OBT result is classified as either positive or negative, dictating the next steps in patient care. A negative result means no occult blood was detected, and average-risk individuals are generally recommended to repeat the screening test at the prescribed interval, usually yearly or every two years.
A positive OBT result indicates blood was found in the stool, but this is a screening result, not a definitive diagnosis of cancer. The bleeding source could be a benign condition, such as hemorrhoids, diverticulosis, or peptic ulcers. Because the OBT cannot identify the precise cause or location, a follow-up diagnostic procedure is necessary.
The standard follow-up test for a positive OBT is a colonoscopy. This procedure allows a physician to visually inspect the entire colon and rectum, identify the source of the bleeding, and remove precancerous polyps or take biopsies. The OBT acts as a flag to identify the smaller group requiring this more invasive, definitive investigation.