Cologuard offers a non-invasive, at-home screening method for detecting colorectal cancer. It serves as a tool for early detection, designed for individuals at average risk, providing a convenient screening option.
How Cologuard Works
Cologuard analyzes a stool sample to detect specific markers associated with colorectal cancer or precancerous conditions. The test identifies altered DNA and checks for the presence of blood in the stool, which can indicate bleeding in the gastrointestinal tract.
This multi-target approach allows Cologuard to identify subtle changes that might signal the presence of abnormal growths. The test is a screening tool, meaning it helps identify individuals who might need further investigation. It is not a diagnostic test, so a positive result does not confirm a cancer diagnosis.
Understanding a Positive Cologuard Result
A positive Cologuard result indicates that the test detected altered DNA and/or hemoglobin biomarkers in the stool. These markers are associated with colorectal cancer or advanced adenomas, but a positive result does not definitively mean cancer is present.
The test’s sensitivity for detecting colorectal cancer is high, with studies showing it can find approximately 92% of colon cancers. However, the specificity, which indicates the test’s ability to correctly identify individuals without the disease, is lower. Cologuard has an overall specificity of around 87%, meaning that a certain percentage of positive results will occur in individuals who do not have cancer or advanced precancerous lesions.
This leads to a false positive rate, where the test yields a positive result even when no cancer or advanced precancerous polyps are found upon further examination. In clinical studies, approximately 13% of individuals without colorectal cancer or advanced adenomas received a false positive Cologuard result. The positive predictive value (PPV) for colorectal cancer, the likelihood that a positive test truly indicates cancer, has been reported around 42%. This means that for every 100 positive Cologuard tests, about 42 might genuinely indicate cancer, while the remaining 58 could be false positives.
Reasons for a Positive Result
A positive Cologuard result can stem from several biological factors, not all of which indicate cancer. The test detects genetic material and blood that can be shed by abnormal cells in the colon. While this can include DNA changes from colorectal cancer or advanced precancerous polyps, other conditions can also cause these markers to appear.
For instance, non-cancerous conditions such as hemorrhoids, inflammation, or other gastrointestinal issues can lead to the presence of blood in the stool. These conditions can trigger a positive result because the test identifies the blood, even if it is not related to cancer or precancerous growths. Similarly, certain non-advanced adenomas or benign growths may shed altered DNA or blood, contributing to a positive test.
The presence of any of these factors can contribute to a Cologuard test flagging a positive result. A positive finding suggests the need for further investigation to determine the exact cause of the detected markers, as the test acts as a screening signal rather than a definitive diagnosis.
Following Up on a Positive Result
Upon receiving a positive Cologuard test result, the next required step is a diagnostic colonoscopy. This procedure allows a direct visual examination of the entire large intestine. A colonoscopy can confirm or rule out the presence of colorectal cancer or precancerous polyps.
During a colonoscopy, a healthcare provider can identify and remove any suspicious polyps or abnormal tissues. These are then sent for laboratory analysis to determine if they are cancerous. This immediate removal of polyps during the procedure is a significant advantage of colonoscopy, as it can prevent them from developing into cancer.
Many positive Cologuard results do not ultimately lead to a cancer diagnosis after a colonoscopy. Often, the follow-up colonoscopy reveals no cancer or only benign conditions. As of 2023, Medicare and many commercial insurance plans cover the cost of a follow-up colonoscopy after a positive Cologuard result, recognizing it as a continuation of the screening process.