Non-invasive screening methods, such as the multi-target stool DNA test Cologuard, offer a convenient option for colorectal cancer detection. This at-home tool analyzes a stool sample for specific biomarkers associated with cancer and precancerous growths. A common question is whether pre-existing gastrointestinal conditions, like diverticulitis, can interfere with the test’s accuracy. Understanding the mechanisms of both the test and the condition explains why diverticulitis may lead to a positive result, even without cancer.
How Cologuard Detects Colorectal Cancer Markers
The Cologuard test uses a dual-pronged approach to identify potential signs of colorectal cancer and advanced precancerous polyps. One part of the analysis detects altered human DNA shed from the colon lining into the stool sample. Specifically, the test targets gene mutations, such as those in the KRAS gene, and epigenetic markers like the methylation patterns of the NDRG4 and BMP3 genes, which are associated with cancer development.
The second component is an immunoassay designed to detect human hemoglobin. Hemoglobin, the protein in red blood cells, indicates bleeding in the lower gastrointestinal tract. This microscopic or occult blood can signal a cancerous tumor or a large precancerous polyp, as these growths often bleed easily. The test is highly sensitive to hemoglobin, regardless of the bleeding source.
Diverticulitis and Intestinal Inflammation
Diverticulitis involves the inflammation or infection of small, bulging pouches, called diverticula, that form in the lining of the colon. The mere presence of these pouches is diverticulosis, which is common, especially in individuals over 60. When these pouches become inflamed or irritated, the condition progresses to diverticulitis.
The inflammation or injury associated with diverticulitis can damage small blood vessels near the diverticula. This vascular injury causes blood to pass into the colon and mix with the stool. While bleeding can sometimes be visible, it is often microscopic or occult, meaning it is not apparent to the naked eye. The presence of this occult blood, a frequent consequence of active diverticular disease, creates an overlap with the markers Cologuard is designed to find.
The Connection: Why Diverticulitis Can Affect Test Results
The fundamental issue is that the Cologuard test, while sophisticated in detecting DNA alterations, cannot distinguish the source of the detected blood. Since the test includes a highly sensitive screen for hemoglobin, any condition that causes gastrointestinal bleeding, including active diverticulitis or bleeding diverticulosis, can trigger a positive result. This leads to a false positive, where the test indicates a potential problem (like cancer) that is not actually present.
A positive Cologuard result is determined by a proprietary algorithm that weights the findings from both DNA markers and hemoglobin detection. Even if cancer-specific DNA markers are negative, a high level of occult blood from a non-cancerous source like diverticulitis can be enough to push the overall result into the positive range. Approximately 13% of patients without colorectal cancer or advanced polyps receive a false positive result from the test.
This high rate of false positives highlights the test’s sensitivity to blood, which is a design feature meant to detect the earliest signs of malignancy. However, this also means that benign conditions causing bleeding, such as inflammation from diverticulitis, are frequently flagged.
Next Steps After a Positive Cologuard Result
A positive Cologuard result should never be dismissed, even if a non-cancerous cause like diverticulitis is suspected. Established medical guidelines require that any positive non-invasive stool test necessitates a follow-up diagnostic colonoscopy. This procedure is the gold standard for visual inspection and definitive diagnosis.
The colonoscopy allows a gastroenterologist to examine the entire lining of the colon, confirming the presence of diverticula and determining if they are the source of bleeding. Crucially, the procedure also allows for the detection and immediate removal of any precancerous polyps or cancerous lesions that may be present.
Patients should aim to schedule this necessary diagnostic colonoscopy within three months of receiving the positive result, and no later than six months. Before the procedure, communicate all existing gastrointestinal conditions, including any history of diverticulitis, to the physician. This information helps the medical team interpret the Cologuard result, but it does not negate the requirement for the colonoscopy.