The Fecal Immunochemical Test (FIT) is a non-invasive screening tool used to detect early signs of colorectal cancer (CRC). It identifies tiny, non-visible amounts of human hemoglobin, a protein found in red blood cells, in a stool sample. The presence of this blood may indicate bleeding from the lower digestive tract, a common symptom of polyps or cancer. Unlike older stool-based tests, the FIT uses specific antibodies that react only to human blood, enhancing its specificity.
Measuring the FIT Test Performance
The accuracy of any medical screening test is measured using two statistical concepts: sensitivity and specificity. Sensitivity refers to the test’s ability to correctly identify individuals who truly have the disease (the true positive rate). For detecting established colorectal cancer, the FIT test demonstrates high sensitivity, often ranging from 74% to over 90%, depending on the specific test and the positivity threshold used.
Specificity, conversely, measures the test’s ability to correctly identify individuals who do not have the disease (the true negative rate). The FIT test is notably specific, typically showing a specificity of 90% or higher. This means it rarely produces a false positive result, which is a major advantage as it reduces the number of people who must undergo unnecessary follow-up colonoscopies.
A significant limitation of the FIT is its lower sensitivity for detecting pre-cancerous growths, specifically advanced adenomas or polyps. While highly effective for cancer, the sensitivity for these advanced adenomas often falls into a much lower range, sometimes as low as 30% to 40%. This difference occurs because pre-cancerous polyps may bleed less frequently or profusely than an established tumor, making detection more challenging.
Variables That Affect Test Reliability
The reliability of a FIT result can be influenced by several external and procedural factors. Colorectal lesions, including both polyps and cancers, often bleed intermittently rather than constantly. Therefore, a person with a bleeding lesion might have a negative result if the sample was collected during a period of non-bleeding.
Intermittent bleeding necessitates annual testing for FIT-based screening programs. Consistent, repeated testing over time increases the chance of catching a bleeding episode and detecting a lesion. Procedural errors, such as improper sample collection or delays in shipping the sample, can also compromise accuracy. The hemoglobin protein the test detects degrades over time, potentially leading to a false negative result if the sample is not handled promptly.
The FIT’s high specificity is aided because it detects only human hemoglobin, meaning it is not affected by dietary factors like red meat, unlike older chemical tests. However, non-cancer related bleeding in the lower gastrointestinal tract can still cause a false positive result. Conditions such as hemorrhoids, anal fissures, or inflammatory bowel disease (IBD) may cause blood in the stool, leading to a positive result even when cancer is not present.
Interpreting Positive and Negative Results
Understanding the meaning of a positive or negative FIT result is crucial for appropriate follow-up care. A positive result indicates that blood was detected in the stool sample, which mandates further investigation. This result does not confirm a cancer diagnosis, as the bleeding could be due to other benign causes like ulcers, hemorrhoids, or IBD.
The next step after a positive FIT is a diagnostic colonoscopy. This procedure allows a physician to visually inspect the entire colon and identify the source of the bleeding. A colonoscopy is necessary to determine if the blood comes from a polyp, a cancerous tumor, or a less serious condition, strongly indicating the need for this definitive diagnostic procedure.
Conversely, a negative FIT result means that no blood was detected in the specific sample provided. A negative result provides reassurance, suggesting a low probability of having colorectal cancer, but it is not a guarantee that the colon is clear of disease. Because lesions bleed intermittently, a negative result may represent a false negative (a scenario where a polyp or cancer is present but did not bleed during collection). This possibility underscores why adhering to the recommended annual screening schedule is important, as consistent, yearly testing helps prevent “interval cancer.”