A positive result from a Cologuard test can be unsettling, but it signals the need for further investigation. Cologuard is a non-invasive screening tool for colorectal cancer and advanced precancerous polyps, designed for adults at average risk. A positive finding indicates that a more definitive diagnostic procedure is necessary to determine the source of the abnormal results.
What Cologuard Actually Detects
The Cologuard test analyzes a stool sample for two main categories of biomarkers associated with colorectal neoplasia. One category is the presence of occult hemoglobin, a protein found in red blood cells. This hidden blood is often not visible and can indicate bleeding in the lower gastrointestinal tract, potentially from a tumor or large polyp.
The second category involves detecting altered DNA markers shed from cells lining the colon and rectum. As precancerous polyps or cancerous tumors grow, they continually shed cells into the stool containing specific genetic mutations. Cologuard looks for changes, such as hyperplastic alterations and mutations in genes like KRAS, BMP3, and NDRG4. These DNA changes are indicators of abnormal cellular growth.
The test combines the results of the DNA analysis and hemoglobin detection into a single report. The presence of these biomarkers indicates cellular changes, not a definitive diagnosis of cancer. A positive result means enough altered DNA or blood markers were found to flag the need for a follow-up procedure.
Interpreting the Positive Result
A positive Cologuard result should be viewed as a screening flag, not an immediate cancer diagnosis. The test is highly effective at detecting colorectal cancer, with a sensitivity of around 92% across all stages. However, its ability to detect advanced precancerous polyps is lower, at about 42%.
The positive result indicates an elevated chance of finding invasive cancer or advanced precancerous lesions during a follow-up examination. Clinical studies show that for every 100 people with a positive result, approximately three will have colorectal cancer. Additionally, around 34 out of 100 will be found to have advanced precancerous polyps (APLs).
The detection and removal of these APLs is important, as they are the growths most likely to develop into cancer over time. The remaining positive results often lead to findings of non-advanced polyps, benign conditions, or no neoplastic findings. While a positive result is concerning, it signals a significant chance of having an abnormality that requires attention.
The Required Diagnostic Colonoscopy
Following a positive Cologuard test, the required next step is a diagnostic colonoscopy. This procedure is the appropriate follow-up because Cologuard is strictly a screening test and cannot provide a visual diagnosis. Colonoscopy is the standard procedure as it is both diagnostic and therapeutic.
The procedure involves a complete bowel preparation, usually followed by sedation. A doctor uses a flexible tube with a camera to examine the entire lining of the colon and rectum. The goal is to visually locate the source of the DNA and hemoglobin markers that triggered the positive Cologuard result.
During the colonoscopy, any polyps or abnormal tissue found can be removed immediately, preventing them from developing into cancer. Removed tissue is sent for a biopsy to determine if the growth is benign, precancerous, or cancerous. Do not postpone this follow-up, as delaying a colonoscopy beyond six months significantly increases the risk of being diagnosed with advanced-stage disease.
Understanding Test Accuracy
Understanding the concepts of false positives and false negatives provides a complete picture of Cologuard’s performance. A false positive occurs when the test returns a positive result, but the subsequent colonoscopy finds no cancer or advanced precancerous lesions. The false positive rate for Cologuard is around 13%.
Conversely, a false negative occurs when the Cologuard test is negative, but cancer or an advanced lesion is present. While the false negative rate for cancer is low, the test’s lower sensitivity for advanced polyps means some growths may be missed.
Although the possibility of a false positive can cause anxiety and lead to an unnecessary procedure, the test’s high sensitivity for cancer makes the follow-up colonoscopy a necessary response. Cologuard is a valuable screening option, especially for individuals who might otherwise avoid screening.