Colorectal cancer is highly treatable when detected early. Non-invasive screening methods, such as the Cologuard test, encourage more people to undergo testing. Cologuard analyzes a stool sample for signs of disease. A common question is whether the test can identify precursor lesions, specifically polyps, in addition to cancer.
Understanding the Cologuard Test
The Cologuard test is a non-invasive, prescription-based screening tool for adults at average risk for colorectal cancer. It is FDA-approved for individuals typically starting at age 45 who do not have symptoms or a family history indicating higher risk. The test is convenient, performed at home without dietary restrictions or bowel preparation. Cologuard is a screening method intended only to identify those who may need further diagnostic evaluation, and it does not replace a colonoscopy for high-risk patients.
How Cologuard Detects Abnormalities
The test employs a multi-target approach, analyzing the stool sample for two main types of biomarkers. The first component detects occult blood, specifically human hemoglobin, which suggests bleeding in the lower gastrointestinal tract. This microscopic blood loss can indicate the presence of a lesion, such as a polyp or a tumor.
The second component analyzes altered human DNA shed into the stool by colon cells. Cologuard targets specific DNA markers, including mutations in the KRAS gene and methylation markers like NDRG4 and BMP3. These genetic changes are associated with colorectal cancer and advanced precancerous lesions. An algorithm combines the results of the DNA analysis and hemoglobin detection to generate the overall result.
The Role of Colorectal Polyps
Colorectal polyps are growths on the inner lining of the colon or rectum, typically beginning as small, noncancerous lesions. While many polyps are benign, others, known as adenomas, are considered precancerous and can develop into cancer. Advanced adenomas are the most concerning type because they carry a higher risk of malignant transformation.
As these abnormal growths increase in size, they shed cells and blood into the intestinal lumen. The DNA shed by advanced adenomas often contains the same genetic mutations and altered methylation patterns that Cologuard detects. The physical shedding of these mutated cells and accompanying microscopic bleeding make them detectable by this screening method.
Answering the Key Question: Polyps and Positive Results
Yes, polyps, particularly advanced adenomas, frequently cause a positive Cologuard result. The test is designed to detect the DNA and blood biomarkers shed by these precancerous lesions. Clinical studies show the test is highly effective at identifying colorectal cancer, with a sensitivity around 92% to 94%.
The test’s ability to detect advanced precancerous lesions, including advanced adenomas, is also significant, with sensitivity values ranging from 42% to 45%. This means Cologuard identifies a substantial portion of high-risk polyps that could turn into cancer. A positive result often indicates an advanced adenoma rather than cancer, highlighting the test’s utility in early detection.
Among individuals with a positive Cologuard result, a large percentage will be found to have an advanced precancerous lesion upon follow-up colonoscopy. This demonstrates the test functions by flagging significant abnormalities requiring further investigation. However, the test can yield a false positive, occurring in about 13% of patients without colorectal cancer or advanced adenomas.
Next Steps After a Positive Screening
A positive Cologuard result signifies that abnormal DNA markers or blood were found, mandating follow-up to determine the source. The screening process is incomplete until a diagnostic procedure is performed. The recommended next step is always a colonoscopy, which is the definitive diagnostic tool.
The colonoscopy allows a gastroenterologist to visually examine the entire colon and rectum, locate the source of the positive result, and immediately remove any polyps or abnormal tissue. This procedure serves both a diagnostic and therapeutic purpose, as removing precancerous polyps prevents them from developing into cancer. Follow-up should be scheduled promptly, ideally within three months, to mitigate risks associated with delayed diagnosis.