The Fecal Immunochemical Test (FIT) is a common, convenient, at-home screening tool used to check for signs of colorectal health issues. It is designed to detect hidden blood in the stool, which can indicate potential problems in the lower digestive tract. A positive FIT result often causes concern, but it is simply an alert requiring further investigation. This information explains what a positive result means and outlines the necessary next steps.
Interpreting a Positive Fecal Immunochemical Test
The FIT is a specific stool test that uses antibodies to measure human hemoglobin, a protein found in red blood cells. Because it targets human hemoglobin, the test is not affected by dietary restrictions, such as eating red meat, or by most medications, unlike older stool tests. The FIT is focused on detecting blood originating in the lower gastrointestinal tract, specifically the colon and rectum.
A positive result means blood was detected in the stool sample, indicating bleeding is occurring somewhere in the digestive tract. The FIT is a screening test, not a diagnostic one, and is not a diagnosis of cancer. The vast majority of people with a positive FIT result do not have colorectal cancer. The test serves as a red flag signaling that a more definitive diagnostic procedure is required to find the source of the bleeding.
Common Reasons for a Positive Result
A positive FIT result signals the presence of blood, which can originate from a wide range of conditions, many of which are not life-threatening. While colorectal cancer is the most serious concern, the positive predictive value for cancer in a screening population is relatively low, often around 3%. The test is also designed to identify precancerous growths, such as advanced adenomas or polyps, which are small cell masses on the colon lining that can bleed.
These precancerous polyps and early-stage cancers often have fragile blood vessels that are easily damaged by passing stool, releasing trace amounts of blood. Finding and removing these polyps is a primary goal of the follow-up investigation, as their removal prevents cancer from developing. Benign, or non-cancerous, conditions are responsible for the majority of positive results.
Common benign causes include hemorrhoids (swollen veins in the rectum or anus) and anal fissures (small tears in the anal lining). These conditions frequently cause bleeding that may not be visible. Other gastrointestinal issues can also lead to a positive result, such as inflammatory bowel disease (IBD), diverticulitis, or ulcers in the lower tract.
Certain medications, particularly blood thinners and regular aspirin use, can increase the odds of a positive result by causing minor gastrointestinal bleeding. Lifestyle factors, such as smoking, increased age, and higher body mass index, are also associated with a greater chance of a positive FIT result. Because the FIT cannot differentiate between these diverse causes, a follow-up procedure is necessary to pinpoint the exact source of the blood.
Mandatory Follow-Up Procedures
The mandatory next step after receiving a positive FIT result is a diagnostic colonoscopy. This procedure is the gold standard for investigating the lower digestive tract and is the only way to determine the cause of the bleeding. A colonoscopy involves a gastroenterologist inserting a flexible tube equipped with a camera into the rectum to visualize the entire colon.
The primary purpose of the colonoscopy is to look for polyps or other abnormalities throughout the large intestine. If polyps are found, the physician can typically remove them during the same procedure, a process called a polypectomy. Tissue samples, or biopsies, can also be taken from suspicious areas for laboratory analysis, providing a definitive diagnosis.
Timely scheduling of the follow-up colonoscopy is important. Studies suggest that delaying the procedure beyond six months can be associated with a higher risk of diagnosing advanced-stage colorectal cancer. Many healthcare systems aim for the follow-up to occur within three months of the positive FIT result to ensure early detection and intervention.
A successful colonoscopy requires thorough preparation, which involves following a clear liquid diet and drinking a prescribed laxative solution. This preparation is essential to clean out the colon so the physician has a clear view of the intestinal lining. Patients are generally given sedation for the procedure and must arrange for transportation home afterward.
In rare cases where a colonoscopy is medically contraindicated due to severe health issues, alternative diagnostic imaging tests may be considered. However, alternatives like computed tomography (CT) colonography do not allow for the removal of polyps during the procedure. For the vast majority of people with a positive FIT, the diagnostic colonoscopy is the only recommended pathway to identify the bleeding source and treat any precancerous findings.