Cologuard is a stool-based screening test that detects colorectal cancer and precancerous growths by looking for abnormal DNA and traces of blood in a stool sample you collect at home. It combines two different detection methods in a single test: it analyzes genetic markers shed by cancer cells into the digestive tract, and it checks for hidden blood that can signal a tumor or polyp. The kit ships to your home, you collect a sample, and mail it to a lab for analysis.
What the Test Actually Detects
Cancer cells and precancerous polyps growing in the colon constantly shed their DNA into stool as the lining of the intestine renews itself. Cologuard targets specific genetic changes that are hallmarks of colorectal cancer. The test looks at three categories of markers simultaneously.
First, it checks for changes called methylation in two genes (NDRG4 and BMP3) that normally act as tumor suppressors. In healthy cells, these genes help regulate growth. When they become silenced through methylation, a chemical process that effectively switches genes off, cells can start growing out of control. Finding high levels of methylation in these genes is a strong signal that abnormal tissue is present in the colon.
Second, the test screens for mutations in a gene called KRAS, which provides instructions for a protein involved in cell division. Specific point mutations in KRAS are commonly found in colorectal cancers and adenomas. Cologuard checks seven of these mutations across two key regions of the gene.
Third, the test measures the total amount of human DNA in the stool sample using a reference gene. A higher-than-normal quantity of human DNA can indicate that cells are being shed more rapidly than usual, which happens when a tumor or large polyp is present.
Layered on top of all this DNA analysis, Cologuard includes an immunochemical blood test. This component uses antibodies to detect tiny amounts of hemoglobin, the oxygen-carrying protein in red blood cells. Tumors and larger polyps in the colon tend to bleed, often in amounts too small to notice. This blood test catches what the naked eye cannot.
How Accurate the Results Are
In clinical trials, Cologuard detected 92.3% of colorectal cancers. That means roughly 9 out of 10 cancers present in the colon were caught by the test. Its specificity, the ability to correctly identify people who don’t have cancer, is about 87% to 90% depending on how the calculation accounts for precancerous findings.
The test is less sensitive for precancerous polyps than for established cancers. This is partly because early-stage growths shed fewer abnormal cells and bleed less frequently. Still, catching even some of these growths early is valuable because removing them during a follow-up colonoscopy prevents cancer from developing in the first place.
False positives are a real consideration. One real-world study of 385 patients with positive Cologuard results found that 61.5% had no significant findings when they followed up with a colonoscopy. Only about 38.5% of positive results corresponded to meaningful lesions. A positive result does not mean you have cancer. It means something triggered the test’s markers and a colonoscopy is needed to find out what’s going on.
Collecting and Shipping Your Sample
The kit arrives in a box containing a plastic bracket, a sample container, a small tube with a probe, a bottle of liquid preservative, and shipping labels. You place the bracket across the toilet bowl under the seat, set the open container into the bracket, and have a bowel movement directly into the container. You’ll want to keep urine out of the container and avoid putting toilet paper in it.
After the sample is collected, you use the probe from the tube to scrape the surface of the stool, then seal the probe back into the tube. Next, you pour the entire bottle of preservative liquid into the container with the stool and seal the lid tightly. That preservative is critical. It stabilizes the DNA in your sample and neutralizes bacterial enzymes that would otherwise break down the genetic material during transit. Early versions of stool DNA tests didn’t include stabilizing buffers and had much lower detection rates as a result. Modern buffers dropped the detection threshold from 1% of mutated DNA copies down to less than 0.1%.
You label both the tube and the container with your name, birthdate, and collection date, then pack everything into the provided box and ship it via UPS. The sample needs to arrive at the lab within three days, so plan your collection day accordingly.
What Happens After a Positive Result
A positive Cologuard result requires a follow-up colonoscopy. This is not optional in any practical sense. The stool test identifies that something abnormal may be present, but only a colonoscopy can confirm what it is, whether that’s cancer, a precancerous polyp, or nothing significant at all.
Timing matters. Research suggests that completing the follow-up colonoscopy within six months minimizes the risk of any lesion progressing, and getting it done within three months provides an additional safety margin. Despite this, adherence is a known problem. One study found that only 44% of patients with a positive Cologuard result actually completed a follow-up colonoscopy.
Screening Schedule and Age Range
The U.S. Preventive Services Task Force recommends colorectal cancer screening for all adults aged 45 to 75. For adults 76 to 85, screening is more selective and depends on individual health factors and prior screening history. Cologuard falls into the “stool DNA-FIT” category, recommended every one to three years. That’s more frequent than colonoscopy (every 10 years) but less frequent than a standalone annual blood-in-stool test.
Who Should Not Use Cologuard
Cologuard is designed for average-risk adults. Several conditions disqualify you from using it, and in these cases your doctor will typically recommend colonoscopy or another surveillance approach instead:
- Personal history of colorectal cancer or advanced precancerous polyps. If you’ve already had these, you need direct visualization of the colon on a more tailored schedule.
- Family history of colorectal cancer in a first-degree relative (parent, sibling, or child) diagnosed at any age.
- Inflammatory bowel disease, including Crohn’s disease or ulcerative colitis, both of which independently raise colorectal cancer risk.
- Hereditary cancer syndromes such as Lynch syndrome or familial adenomatous polyposis, which carry significantly elevated risk and require closer monitoring.
- A recent positive result from another screening test, since the appropriate next step in that case is colonoscopy rather than another stool-based test.
If none of these apply and you’re in the recommended age range, Cologuard offers a way to screen for colorectal cancer without bowel preparation, sedation, or a trip to a clinic. It catches the vast majority of cancers present, though its higher false-positive rate means some people will undergo a colonoscopy they didn’t strictly need. For many, that tradeoff is preferable to skipping screening altogether.