The Fecal Immunochemical Test (FIT) is a non-invasive screening tool used primarily to detect signs of colorectal cancer. It checks for microscopic amounts of blood in a stool sample, which can be an early indicator of disease. Irritable Bowel Syndrome (IBS) is a common gastrointestinal disorder characterized by abdominal pain, bloating, and changes in bowel habits. Many people with IBS symptoms worry that their bowel issues might lead to a positive FIT result. This article clarifies the distinct nature of the FIT test and IBS to address whether functional symptoms can trigger a positive screening result.
How the FIT Test Works
The FIT test specifically identifies the presence of human hemoglobin, the oxygen-carrying protein found in red blood cells. Unlike older chemical tests, the FIT uses antibodies that bind only to human hemoglobin, making it highly specific for detecting lower gastrointestinal bleeding. This immune-based detection means the test is not affected by dietary restrictions, such as red meat consumption, or by most medications.
The test is quantitative, measuring the actual concentration of hemoglobin in the stool sample. A positive result occurs when the detected level of hemoglobin exceeds a predetermined cut-off threshold. Because the test detects intact hemoglobin, it primarily targets blood originating from the lower digestive tract, such as the colon and rectum.
The Distinction Between IBS and Bleeding
Irritable Bowel Syndrome is classified as a functional gastrointestinal disorder, meaning it involves disturbances in gut function, including motility and sensation, without visible structural damage. The symptoms of IBS, such as cramping, diarrhea, and constipation, arise from problems with the gut-brain axis and visceral hypersensitivity. The syndrome itself does not cause inflammation or tissue erosion in the digestive lining that leads to bleeding.
Since a positive FIT result requires the detection of blood, IBS alone does not possess the mechanism to trigger a positive test. IBS is defined by a lack of underlying organic disease, including the absence of structural defects like ulcers, polyps, or tumors that commonly cause microscopic bleeding. Therefore, a person with a confirmed IBS diagnosis should not expect a positive FIT result based on their functional disorder symptoms alone. The appearance of blood in the stool in a person with IBS is considered a “red flag” symptom, suggesting the underlying cause is something other than IBS itself.
Common Non-IBS Reasons for a Positive FIT
A positive FIT result indicates bleeding in the lower digestive tract and mandates further investigation, regardless of a prior IBS diagnosis. The test cannot distinguish between serious and benign sources of bleeding, which is why a positive result requires a follow-up procedure. The primary concern the test aims to identify is colorectal cancer or precancerous polyps, as these growths often have fragile blood vessels that bleed small amounts of blood.
Many other conditions can also cause a positive FIT result:
- Benign sources like hemorrhoids and anal fissures (tears in the lining of the anus).
- Inflammatory Bowel Disease (IBD), including Crohn’s disease and ulcerative colitis, which involves chronic inflammation and mucosal damage.
- Diverticulosis, which involves small pouches forming in the colon wall.
- Certain medications, such as nonsteroidal anti-inflammatory drugs (NSAIDs).
Mandatory Follow-up After a Positive Result
Receiving a positive FIT result means blood was detected, overriding any assumption that symptoms are solely due to Irritable Bowel Syndrome. A positive screening test signals the need for a diagnostic procedure to locate the source of the hemorrhage. The standard next step is a colonoscopy, which allows a healthcare provider to visually examine the entire colon lining.
This procedure is essential to rule out serious conditions like colorectal cancer and to identify and potentially remove precancerous polyps. Even if the patient suspects a benign cause, such as known hemorrhoids, a colonoscopy is still required because the FIT test cannot pinpoint the location or cause of the bleeding. Delaying this follow-up can increase the risk of a late-stage cancer diagnosis.