Colorectal cancer screening programs use non-invasive methods, such as stool-based tests, to detect early signs of disease in people who have no symptoms. Among the most widely used options is the Fecal Occult Blood Test (FOBT) or its modern version, the Fecal Immunochemical Test (FIT). A positive result signals the presence of blood not visible to the naked eye, which can cause concern. Understanding what a positive test indicates and the likelihood of different diagnoses is important.
Understanding the Fecal Occult Blood Test
These stool tests detect microscopic traces of blood shed from the lining of the colon or rectum. The term “occult blood” refers to this hidden presence of blood, which is not apparent during a normal bowel movement. The test acts as an initial filter to identify individuals who require further diagnostic evaluation.
The older guaiac-based FOBT used a chemical reaction that could react to blood from certain foods. The modern Fecal Immunochemical Test (FIT) is an improvement because it uses antibodies to specifically detect human hemoglobin protein. This makes the FIT more specific to bleeding in the lower gastrointestinal tract and eliminates the need for dietary restrictions. A positive result confirms blood is present in the stool, making the test sensitive but not specific for any single underlying condition, including cancer.
Interpreting a Positive Result
A positive screening test means the individual has tested positive for hidden blood. The likelihood that a positive FIT result is caused by invasive colorectal cancer is statistically low. This statistical measure is known as the Positive Predictive Value (PPV).
In large-scale screening programs, the percentage of positive FIT results ultimately diagnosed as colorectal cancer typically falls within the range of 2.7% to 11.8%. This indicates that most people with a positive test do not have cancer. The probability of finding a pre-cancerous condition, such as advanced adenomas or polyps, is much higher, often ranging from 19.1% to 30%. These polyps are growths that have the potential to develop into cancer if left untreated.
Non-Cancerous Causes of Occult Blood
The low Positive Predictive Value for cancer indicates that many benign conditions cause intermittent bleeding in the lower digestive tract. A positive FIT result most frequently points to non-malignant sources that are often treatable once identified.
One frequent cause is hemorrhoids, which are swollen veins in the rectum or anus that bleed easily. Anal fissures, small tears in the lining of the anus, also commonly cause minor bleeding. Diverticulosis, where small pouches form in the colon wall, can occasionally lead to bleeding episodes.
Other common non-cancerous sources include inflammatory conditions, such as inflammatory bowel disease (IBD) or peptic ulcers. Certain medications, particularly nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen or aspirin, can also cause minor irritation and bleeding. These conditions cause the microscopic blood found by the FIT, meaning a positive result requires further investigation rather than confirming a cancer diagnosis.
Required Next Steps After a Positive Test
A positive Fecal Immunochemical Test requires diagnostic follow-up to determine the exact source of the occult blood. The next step is a colonoscopy, which is the standard for investigating the lower gastrointestinal tract. This procedure involves a physician using a flexible tube with a camera to visually examine the entire length of the colon and rectum.
The colonoscopy locates the source of the bleeding and determines if it is caused by cancer, a pre-cancerous polyp, or a benign condition. If polyps are found, they can often be removed immediately, preventing them from developing into cancer. Prompt scheduling of this diagnostic procedure is required after a positive test.