Cologuard False Positives: What They Mean and What to Do
A positive Cologuard result does not mean you have cancer, and it does not mean the test made a mistake you can ignore. In the FDA’s data for the newer Cologuard Plus test, about 30 of every 100 positive results turn up no neoplastic finding at colonoscopy. The other 70 do turn up something: cancer, an advanced precancerous lesion, or a smaller adenoma. Either way, the next step is the same: a colonoscopy.
Below, we explain what “false positive” means for a stool DNA test, why the numbers you see online range from 9% to more than 70%, what can trigger a false alarm, and how to handle the weeks after a positive result.
Precision Digestive Healthprecisiondigestive.comFollow Up a Positive Cologuard ResultDr. Meet Parikh provides colonoscopy and specialized digestive care in South Plainfield for patients following up on a positive stool test.Schedule an appointment
TL;DR
- Roughly 30 of every 100 positive Cologuard Plus results have no cancer, precancer or adenoma at colonoscopy, according to the FDA review. About 70 do have a finding.
- Published “false positive” rates vary widely because researchers define a positive colonoscopy differently.
- Bleeding from hemorrhoids, diarrhea and menstrual blood can all cause a false alarm. Tell your doctor about them before you collect a sample.
- Whatever the cause, a positive stool test needs a follow-up colonoscopy. A repeat Cologuard is not a substitute.
- If you are on Medicare, CMS rules treat that colonoscopy as part of the screening. Other plans differ, so check yours.
Table of Contents
- What a false positive means for a stool DNA test
- How often do Cologuard false positives happen?
- What can cause a false alarm
- Why a “false positive” still needs a colonoscopy
- What to do after a positive result
- Perspective: how we think about Cologuard
- Reducing the chance of a false alarm
- When a colonoscopy is the better first choice
- Schedule your follow-up colonoscopy in South Plainfield
- FAQ
- Sources
What a false positive means for a stool DNA test
Cologuard is a multitarget stool DNA test. It looks for altered DNA markers shed by colorectal neoplasia and for hidden blood (occult hemoglobin) in a stool sample. The FDA summary for Cologuard Plus describes it as a qualitative test: the answer is positive or negative, not a risk score.
A false positive is a positive result when the colonoscopy finds no cancer and no advanced precancerous lesion. The test picked up DNA or blood markers, but the cause was not the kind of lesion it was designed to find. Sometimes the cause is harmless and obvious, such as a bleeding hemorrhoid. Sometimes there is no clear explanation.
The term hides a lot. A person with a “false positive” may still have a small adenoma, a polyp that can turn into cancer over years. The word “false” describes the test’s target, not whether anything was there.
How often do Cologuard false positives happen?
The honest answer is that it depends on which test, which study and how you count. Here is how the main numbers fit together.
The FDA figures for Cologuard Plus
The FDA’s review of Cologuard Plus breaks down what colonoscopy found for every 100 people who tested positive:
| Colonoscopy finding | Per 100 positive tests |
|---|---|
| Colorectal cancer | 3 |
| Advanced precancerous lesion | 34 |
| Non-advanced adenoma | 33 |
| No neoplastic findings | 30 |
Those figures come from the FDA’s Cologuard Plus documentation. Read strictly, 30 of 100 positives had nothing neoplastic, and 63 of 100 had no cancer or advanced precancerous lesion but 33 of those still had an adenoma.
Specificity: the number behind the headlines
Specificity is the share of people without advanced neoplasia who correctly test negative. In the pivotal BLUE-C trial, Cologuard Plus showed 90.6% specificity for advanced neoplasia, an improvement on the roughly 87% reported for the original Cologuard. So about 9% to 10% of people without advanced neoplasia can still get a positive result on the newer test, and more on the original.
That is where figures like 9%, 13% and 14% in search results come from. They answer a slightly different question from the one in the FDA table above: “out of healthy people, how many get flagged?” versus “out of people flagged, how many had nothing?”
Why some studies report much higher rates
A 2023 analysis of the New Hampshire Colonoscopy Registry showed how much the definition matters. When a positive colonoscopy was defined narrowly, as cancer or advanced adenoma, the false discovery rate was 71.9%. When it was widened to follow US Multi-Society Task Force guidelines, which include smaller adenomas, it fell to 33.2%.
So a headline claiming most positives are “false” is usually counting small adenomas as a miss. Those adenomas are the kind of finding we remove during colonoscopy precisely because they can progress.
Also keep in mind that real-world studies of small groups, such as those from a single hospital, can report very different rates from the FDA trial. Patient mix, follow-up and definitions all change the result.
What can cause a false alarm
The FDA documentation lists situations that can produce a positive result without a serious lesion. Several involve blood that did not come from a tumor or polyp.
- Hemorrhoids or other rectal bleeding. The test detects hemoglobin, and it cannot tell where the blood came from.
- Diarrhea. The FDA review says patients should not provide a sample while they have diarrhea.
- Menstrual blood. Blood from a period can contaminate the sample.
- Blood in the urine. Urine mixing with the stool sample is also a concern.
- Bleeding cuts or fissures near the anus.
The FDA’s guidance is that people with these issues should discuss them with a healthcare provider rather than simply completing the test. If you have rectal bleeding, ongoing diarrhea or blood in your stool, that is a reason to be evaluated, not to mail a kit.
Other conditions that cause inflammation in the bowel, such as inflammatory bowel disease, can also raise the chance of an abnormal result. If you have a known diagnosis or suspect one, our guide to screening tests for IBD explains what is used instead.
Why a “false positive” still needs a colonoscopy
It is tempting to hear “there is a decent chance it is nothing” and wait. We would not.
The National Cancer Institute’s screening fact sheet is clear that a positive stool-based test calls for a follow-up colonoscopy. There are three reasons:
- Colonoscopy is the diagnostic test. A stool test narrows the field. Only direct visualization of the colon can confirm what is there.
- Colonoscopy is also the treatment. Polyps and adenomas can be removed during the same procedure, so the “follow-up” often prevents cancer rather than just looking for it.
- The positive result is often telling the truth. Across the FDA figures above, 70 of 100 positives had a real neoplastic finding.
Research on repeat testing points the same way. A study of the second round of mt-sDNA testing found a high positive predictive value for colorectal neoplasia, which supports using these tests as a trigger for colonoscopy rather than an endpoint.
What if the colonoscopy is normal?
This is the situation people find most confusing. You tested positive, the colonoscopy looked clean, and nobody can tell you why.
There are a few possibilities. The marker may have come from a source such as a hemorrhoid. A tiny lesion may have been missed or may have been in a hard-to-see fold. Research from the Early Detection Research Network, in a study of patients with an apparent false positive stool DNA test, looked at exactly this question, including whether missed colonic lesions or findings in the upper GI tract could explain it. A related analysis of postcolonoscopy neoplasia detection notes that variability between colonoscopy operators contributes to missed cancers.
What to do next is a conversation to have with your gastroenterologist. It depends on the quality of your bowel prep, how thorough the exam was, your symptoms and your family history. Do not decide on your own to repeat the stool test and treat a negative as the final word.
What to do after a positive result
Here is the sequence we walk patients through.
- Do not panic, and do not wait. Most positives are not cancer, but a positive result needs action in weeks, not months. Real-world data suggest many people never complete the follow-up, and that gap is the real danger.
- Call a gastroenterologist and bring the result. Bring the report, your medication list (especially blood thinners and aspirin) and any symptoms you have had.
- Tell us about bleeding, hemorrhoids and diarrhea. They do not replace the colonoscopy, but they help us interpret what we find.
- Confirm your coverage. Ask your insurer whether the colonoscopy after a positive stool test is billed as screening or diagnostic.
- Prepare properly. A good prep is the single biggest factor in how well the doctor can see. Our colon cancer screening guide covers how to choose and prepare.
- Read your report. After the procedure, our guide to interpreting colonoscopy results explains terms like adenoma, sessile and surveillance interval.
We also have a post on what a positive Cologuard result actually means if you want a shorter walk-through.
Will the colonoscopy be covered?
For Medicare patients, CMS has changed the rules so that a colonoscopy after a positive stool-based screening test is no longer treated as diagnostic, which means it is generally covered as part of the screening continuum, with correct coding by the practitioner. Private plans vary by state and by plan design. Call the number on your insurance card and ask specifically about a follow-up colonoscopy after a positive stool test. We cannot quote a price for you here, because we do not publish one, but our office can help you sort out the billing questions before the procedure.
Perspective: how we think about Cologuard
We are not against stool testing. For someone who will not schedule a colonoscopy, a stool test that gets done is better than screening that never happens. The trouble starts when people misunderstand what the result means.
In our practice, the pattern is predictable. A patient receives a positive result, reads that “13% are false positives” online, and decides to wait or retest. Others assume a positive means cancer and spend a frightening week imagining the worst. Neither reaction serves them.
What we tell patients is simple. A positive Cologuard is a reason to look, not a diagnosis. Dr. Parikh is a board-certified, fellowship-trained gastroenterologist, and the colonoscopy he performs is both the answer and, if a polyp is there, the treatment. We also speak with patients in English, Gujarati, Hindi and Spanish, because we have seen how much a result is misunderstood when it is explained in a second language.
We also want people to understand what a stool test cannot do. It does not look at your symptoms. If you have bleeding, a change in bowel habits, unexplained weight loss or anemia, you should see a gastroenterologist rather than screen. Our post on digestive health red flags lists what should not wait.
— Precision Digestive Health
Reducing the chance of a false alarm
You cannot eliminate false positives, but you can avoid the avoidable ones.
- Check your timing. Do not collect a sample during a period, during a flare of hemorrhoids or with active diarrhea. Wait until it settles, or ask your doctor first.
- Follow the kit instructions exactly. The FDA documentation is explicit about keeping urine and toilet water out of the sample.
- Tell your doctor about known conditions. Hemorrhoids, fissures and inflammatory bowel disease change how a result should be read.
- Ask whether a stool test is right for you. People with a family history of colorectal cancer, prior polyps, or symptoms generally need a colonoscopy rather than a stool test.
General gut habits matter for overall digestive comfort too, and this digestive health guide covers everyday steps. They do not replace screening.
When a colonoscopy is the better first choice
A stool test makes sense for average-risk adults without symptoms who prefer a noninvasive option. A colonoscopy first is the better route when you have a family history, a personal history of polyps or IBD, or symptoms such as bleeding. It is also the better route if you want to avoid the chance of a positive stool test followed by a colonoscopy anyway. If you are still choosing, our overview of the role of colonoscopy in prevention lays out the trade-offs, and why colonoscopies matter for long-term health explains what the exam adds.
Schedule your follow-up colonoscopy in South Plainfield
If you have a positive Cologuard result, or you are deciding how to screen, we can help. Dr. Meet Parikh offers colonoscopy and colon cancer screening at our South Plainfield, NJ practice, along with diagnosis and ongoing care for hemorrhoids, IBD, IBS and other digestive conditions. See everything we offer on our services page, or start at precisiondigestive.com to book an appointment.
FAQ
How common are false positives with Cologuard?
It depends on the version and how you count. For Cologuard Plus, the FDA data show about 30 of every 100 positive results have no neoplastic finding at colonoscopy. Separately, trial specificity for advanced neoplasia was about 90.6%, so roughly 9% to 10% of people without advanced neoplasia may still get a positive result.
What can cause a positive Cologuard test besides cancer?
Blood from hemorrhoids, anal fissures, menstrual bleeding or blood in the urine can trigger the hemoglobin marker, and the FDA advises against providing a sample during diarrhea. Adenomas and advanced precancerous lesions also cause positives, and those are real findings. Inflammatory bowel disease may also be a factor.
Should I repeat the Cologuard if I think my result was a false positive?
No. A positive stool test should be followed by a colonoscopy, not another stool test. A repeat negative could falsely reassure you while a lesion remains in place. If your colonoscopy is normal, ask your gastroenterologist whether any further testing is appropriate.
Will my insurance cover the colonoscopy after a positive Cologuard?
Under Medicare, the follow-up colonoscopy after a positive stool-based test is no longer considered diagnostic, so it is covered as part of screening when coded correctly. Commercial and Medicaid plans vary, so ask your insurer before scheduling. Our office can help you work through the questions to ask.
Sources
- FDA summary of safety and effectiveness data, Cologuard Plus
- What do ‘false-positive’ stool tests really mean? New Hampshire Colonoscopy Registry data (PubMed)
- Cologuard Plus FDA approval and BLUE-C trial results (Patient Care)
- Screening Tests to Detect Colorectal Cancer and Polyps (NCI)
- Multitarget stool DNA testing, second round of testing (PubMed)
- Evaluation of patients with an apparent false positive stool DNA test (EDRN)
- Detection of postcolonoscopy colorectal neoplasia by multi-target stool DNA (PubMed)
- CMS MM13017: colorectal cancer screening coverage
