The detection of hidden blood in the stool is a primary method for non-invasive screening to assess colorectal health. This screening looks for tiny amounts of blood, often called “occult” blood, that are not visible to the naked eye. The presence of this blood can be an early indicator of conditions in the gastrointestinal tract, including precancerous lesions or cancer. Modern screening focuses on globin, a specific protein component of blood, which is a highly reliable marker for identifying potential issues.
What Globin Is and Why It Appears in Stool
Globin is the protein subunit of hemoglobin, the complex molecule within red blood cells responsible for transporting oxygen. When bleeding occurs in the digestive tract, red blood cells enter the intestines and mix with the stool. A modern screening test aims to detect the presence of this globin component.
The location of the bleeding is a key factor in why globin is used as a marker. Globin is susceptible to degradation by the powerful digestive enzymes and acids found in the stomach and small intestine. If bleeding originates in the upper GI tract, the globin protein is largely broken down before reaching the large intestine.
When intact globin is detected in the stool, it strongly suggests the blood source is located in the lower GI tract—specifically the colon or rectum. This makes globin a highly specific marker for conditions like polyps or cancers that bleed in the large intestine. The test measures only intact human globin, ensuring results are not affected by dietary factors.
How the Fecal Immunochemical Test (FIT) Works
The Fecal Immunochemical Test, or FIT, is the primary method used to specifically detect human globin in a stool sample. This test is based on a refined immunological process that utilizes antibodies. The test kit contains antibodies designed to bind exclusively to the human globin protein found in the sample.
When a stool sample containing globin is introduced to the test device, the specific antibodies attach to the protein, triggering a reaction that signals a positive result. This method is highly precise because the antibodies will not react to animal blood or other substances in the stool. This is an improvement over older chemical-based tests, such as the guaiac Fecal Occult Blood Test (gFOBT), which reacted broadly to peroxidase activity, causing false positives from foods like red meat or certain vegetables.
Because the FIT relies on an immune reaction to human globin, patients are not required to follow special dietary restrictions before collecting their sample. This specificity for human blood originating in the lower GI tract makes FIT a preferred and highly accurate screening tool. The process typically involves collecting a small stool sample at home and sending it to a laboratory for analysis.
Understanding Positive and Negative Results
A negative FIT result means the test did not detect globin in the stool sample, suggesting a low likelihood of bleeding from the lower gastrointestinal tract. While reassuring, this outcome does not completely eliminate the possibility of a problem, as some polyps or cancers may not bleed consistently. For this reason, average-risk individuals are advised to repeat the FIT annually to account for the intermittent nature of bleeding.
A positive result indicates that human globin was detected, signaling bleeding somewhere in the lower GI tract. It is important to understand that a positive FIT result does not diagnose colorectal cancer. Bleeding can be caused by various non-cancerous conditions, including hemorrhoids, diverticulitis, or inflammatory bowel disease.
The clinical significance of a positive result is that it acts as a trigger for further diagnostic investigation. The standard follow-up procedure is a colonoscopy, which allows a medical professional to directly visualize the entire colon and rectum. This procedure is necessary to identify the specific source of the bleeding and determine if it is caused by a benign condition, a precancerous polyp, or a cancerous lesion.