Understanding What Foods Cause a False Positive for a Cologuard Test (and Which Ones Don't)

October 23, 2025 •

3 min read

While certain dietary choices can affect older fecal occult blood tests, the makers of Cologuard explicitly state that food and drinks will not affect the results of their stool DNA test. Therefore, understanding what foods cause a false positive for a Cologuard test involves differentiating between testing methods to ensure accurate information and minimize patient anxiety.

Quick Summary

This article clarifies why diet does not impact Cologuard test results, contrasting it with older methods. It details actual factors that can lead to a false positive and explains necessary preparations and precautions for an accurate screening.

Key Points

No Dietary Restrictions: Unlike older tests, the Cologuard test does not require you to change your diet or avoid specific foods before collecting a sample.
Multi-Target Technology: Cologuard analyzes both altered DNA and blood in the stool, making it less susceptible to interference from food.
Older Test Differences: Traditional gFOBT and FIT tests have different mechanisms, and the gFOBT test specifically required avoiding red meat and certain vegetables due to their heme content.
Non-Dietary Factors Matter: False positives on a Cologuard test are more likely caused by medical conditions or other bleeding sources like hemorrhoids or menstruation, not diet.
Positive Result Follow-Up: A positive Cologuard result is not a diagnosis of cancer but indicates the need for a follow-up colonoscopy to investigate further.
High Accuracy: Cologuard is a highly effective, noninvasive screening tool for average-risk individuals, but false positives and negatives can occur.
Proper Timing is Key: Avoid collecting a sample during a period of active bleeding to prevent a potential false positive.

Cologuard and the Modern Screening Method

Cologuard is an FDA-approved, noninvasive stool DNA test used to screen for colorectal cancer in average-risk individuals aged 45 and older. A key distinction of this test is its multi-marker approach, which detects both specific DNA mutations shed from precancerous polyps or cancer cells and occult blood (hemoglobin) in the stool. Its design makes it uniquely resistant to dietary interference, unlike older screening methods that rely solely on chemical reactions.

Unlike a traditional colonoscopy, which requires significant dietary changes and bowel preparation, Cologuard requires no special food restrictions or fasting before collecting a sample. This eliminates the need to worry about what you eat or drink affecting the test's accuracy related to dietary factors. However, certain temporary conditions, such as gastrointestinal bleeding or menstruation, can still introduce blood into the stool and lead to a false positive.

Older Tests vs. Cologuard: The Dietary Factor

To understand why food is a concern for some tests but not Cologuard, it's essential to compare different at-home screening types. The traditional guaiac fecal occult blood test (gFOBT) and the fecal immunochemical test (FIT) operate differently and have varying dietary requirements.

Guaiac Fecal Occult Blood Test (gFOBT)

Older gFOBT tests detect the heme component of hemoglobin using a chemical reaction with a substance called guaiac. Since heme is also found in red meat and the peroxidase enzyme in certain vegetables, a false positive could occur if these foods were consumed shortly before testing. Because of this, patients were previously instructed to avoid foods like red meat, poultry, certain raw fruits, and vegetables for several days before collecting a sample.

Fecal Immunochemical Test (FIT)

FIT tests use antibodies to detect human hemoglobin protein, making them more specific than gFOBT for human blood. For this reason, FIT tests typically do not require dietary restrictions related to food. The primary concerns for a false positive with FIT are medications and conditions that cause bleeding.

Cologuard (FIT-DNA)

Cologuard combines the FIT technology with a multitarget stool DNA test, analyzing for altered DNA from precancerous polyps or cancer, alongside the blood detection component. This dual approach, combined with a different detection process for hemoglobin, means that food and drinks do not affect the results. The key is to avoid collecting a sample during a time of active bleeding from other sources.

Comparison of At-Home Colorectal Cancer Tests


Feature	Cologuard (FIT-DNA)	Fecal Immunochemical Test (FIT)	Guaiac Fecal Occult Blood Test (gFOBT)
Dietary Restrictions	No	No	Yes, avoid red meat and certain raw fruits/vegetables
Test Method	Stool DNA + Hemoglobin	Antibodies against human hemoglobin	Chemical detection of heme
Interval	Every 3 years (if negative)	Annually	Annually (often replaced by FIT)
Prep Required?	No	No, beyond medication modifications	Yes, dietary restrictions
Sensitivity	High for CRC	Lower than Cologuard for CRC	Generally lower

Non-Dietary Factors That Can Cause a False Positive

While food is not a concern for Cologuard, other non-dietary factors can potentially lead to a false positive result by causing non-cancerous bleeding. These include:

Hemorrhoids: Active bleeding hemorrhoids are a common cause of blood in the stool.
Gastrointestinal Issues: Conditions such as inflammatory bowel disease (Crohn's disease or ulcerative colitis), diverticulosis, or recent gastrointestinal infections can cause bleeding.
Menstruation: Women should not collect a sample while menstruating, as it can contaminate the sample with blood.
Certain Medications: Anticoagulants (blood thinners) and nonsteroidal anti-inflammatory drugs (NSAIDs) can increase the risk of bleeding in the digestive tract.
Recent Medical Procedures: A colonoscopy or biopsy performed shortly before the test may cause transient bleeding.

Conclusion

It is a common and understandable concern to question what foods cause a false positive for a Cologuard test, given the dietary restrictions associated with older stool tests. However, the advanced technology of the Cologuard test, which combines stool DNA and hemoglobin detection, has eliminated the need for dietary modifications. Instead of worrying about what you eat, patients should focus on scheduling the test appropriately to avoid sample collection during times of active bleeding from other medical conditions or menstruation. Always follow the specific instructions provided with the test kit and consult your healthcare provider with any questions about preparation or results. A positive result on a Cologuard test does not automatically mean a cancer diagnosis and necessitates a follow-up colonoscopy to determine the cause. For more information, the Exact Sciences website provides detailed resources.

Frequently Asked Questions

No, eating red meat does not cause a false positive on a Cologuard test. Dietary restrictions for red meat were necessary for older fecal occult blood tests (gFOBT) that relied on chemical detection of blood, but Cologuard's technology is not affected by food.

No, you do not need to avoid specific vegetables like broccoli, horseradish, or turnips before a Cologuard test. These restrictions applied only to older screening methods, and Cologuard is not influenced by plant peroxidases or other food components.

Yes, you can continue taking vitamins and supplements, including iron and Vitamin C, before a Cologuard test. These do not affect the accuracy of the Cologuard test results, though they may have interfered with older gFOBT tests.

No, a positive Cologuard result is not a cancer diagnosis. It means the test detected certain biomarkers and/or blood that could be caused by polyps or cancer, and the next step is a follow-up diagnostic colonoscopy to determine the cause.

False positives can be caused by non-cancerous bleeding from medical conditions such as hemorrhoids, diverticulosis, or inflammatory bowel disease (IBD). Menstruation or recent medical procedures can also introduce blood into the stool.

For most average-risk individuals, Cologuard can be a convenient alternative to a colonoscopy for initial screening. However, if the result is positive or if a person is at high risk, a colonoscopy is required for diagnosis and treatment.