The Fecal Immunochemical Test (FIT) is a common non-invasive screening tool used to detect hidden blood in the stool. It is designed for asymptomatic individuals at average risk for colorectal cancer (CRC) as part of a regular screening regimen. A positive result indicates that human blood hemoglobin has been detected, suggesting bleeding in the lower gastrointestinal tract. This finding signals the need for a more detailed investigation. The FIT is an excellent first step in identifying patients who need further diagnostic testing.
Understanding the FIT Test and Positive Results
A positive FIT result is not a diagnosis of cancer. The percentage of positive FIT tests that lead to a colorectal cancer diagnosis is consistently low, typically ranging between 2.6% and 7.8% in large-scale screening programs. This figure, known as the Positive Predictive Value (PPV), reflects the probability that a positive test correctly identifies the presence of the disease. The exact percentage varies based on the test’s cut-off level and the prevalence of the disease in the screened community.
The test is far more likely to detect advanced adenomas, which are pre-cancerous polyps. A much higher percentage of positive FIT results, often ranging from 20% to 40%, will reveal these advanced polyps upon follow-up. Finding and removing these lesions is a primary success of the screening process, as it prevents colorectal cancer from developing.
Non-Cancer Causes of Positive Results
The low percentage of positive results that are ultimately cancer occurs because many non-malignant conditions cause microscopic bleeding in the digestive tract. These benign issues are the source of most positive FIT results. One of the most frequent causes is hemorrhoids, which are swollen veins in the anus and lower rectum that can bleed easily.
Other common structural issues include anal fissures (small tears in the lining of the anus) and diverticulosis (small pouches developing in the wall of the colon). Inflammatory conditions, such as colitis or Crohn’s disease, can also cause inflammation and bleeding within the gastrointestinal tract. Additionally, certain medications, notably nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin and various blood thinners, increase the risk of minor bleeding.
The Essential Follow-Up Procedure
A positive FIT result necessitates a prompt diagnostic colonoscopy to determine the precise source of the bleeding. The FIT is strictly a screening test, meaning its purpose is only to indicate the possible presence of disease; it cannot provide a definitive diagnosis. The colonoscopy is the mandatory next step because it allows a physician to visually inspect the entire colon and rectum.
During the procedure, the gastroenterologist examines the colon lining for abnormalities, such as polyps or cancerous lesions. If a polyp is found, it can be removed immediately through a polypectomy, which is the most direct way to prevent colorectal cancer. Tissue samples (biopsies) can also be taken from suspicious areas to be analyzed for cancer cells.