Colon cancer is found through two main paths: routine screening before symptoms appear, or diagnostic testing after symptoms like blood in the stool or unexplained weight loss prompt a visit to the doctor. Screening is the far better scenario because it can catch cancer early or even prevent it entirely by removing precancerous polyps before they turn malignant. Current guidelines recommend that average-risk adults begin screening at age 45.
Screening Before Symptoms Appear
Most colon cancers develop slowly from polyps, small growths on the inner lining of the colon that can take 10 to 15 years to become cancerous. That long window is what makes screening so effective. You have several options, ranging from simple at-home stool tests to a full colonoscopy, and all are considered acceptable by the U.S. Preventive Services Task Force.
The recommended screening strategies and their intervals are:
- Stool-based tests: A fecal immunochemical test (FIT) every year, or a stool DNA-FIT test (sold as Cologuard) every one to three years
- Visual exams: Colonoscopy every 10 years, CT colonography (virtual colonoscopy) every five years, or flexible sigmoidoscopy every five years
- Combination approach: Flexible sigmoidoscopy every 10 years paired with an annual FIT
These recommendations apply to adults with no prior history of colon polyps, inflammatory bowel disease, or inherited conditions like Lynch syndrome. If you have a family history of colon cancer or a known genetic predisposition, your doctor will likely recommend starting earlier and screening more frequently.
Stool Tests: The At-Home Option
Stool-based tests work by detecting traces of blood or abnormal DNA shed by tumors and polyps into your stool. They’re noninvasive, require no prep or sedation, and can be done at home. A FIT test detects colorectal cancer with about 79% accuracy and has a false positive rate of roughly 5%, meaning it occasionally flags a problem that isn’t there. The stool DNA-FIT test (Cologuard) is more sensitive but comes with a higher false positive rate of about 13%.
A positive stool test doesn’t mean you have cancer. It means something abnormal was detected and you’ll need a follow-up colonoscopy to find out what’s going on. That’s an important distinction: stool tests are a first step, not a final answer.
Colonoscopy: The Most Thorough Exam
A colonoscopy is the gold standard for finding colon cancer because it lets a doctor see the entire colon and act on what they find in real time. During the procedure, a long, flexible tube with a tiny video camera on the tip is guided through the rectum and the full length of the colon. The doctor inflates the colon with air or carbon dioxide to get a clearer view of the lining.
If the doctor spots polyps, they can remove them on the spot using instruments passed through the scope. They can also take tissue samples, called biopsies, from anything that looks suspicious. Those samples go to a pathology lab where they’re examined under a microscope to determine whether cancer cells are present.
The trade-off for this thoroughness is preparation. You’ll need to follow a clear liquid diet the day before the procedure, limited to things like water, broth, clear juices without pulp, plain coffee or tea, and gelatin. Starting in the afternoon or evening before your procedure, you’ll drink a bowel prep solution in two doses, the second one six to eight hours before the procedure. You’ll need to stop all liquids four hours beforehand. The procedure itself requires sedation, so you’ll need someone to drive you home.
Other Visual Screening Methods
CT colonography, sometimes called a virtual colonoscopy, uses a CT scanner to create detailed images of the colon without inserting a scope. It carries essentially no risk of bleeding or perforation, doesn’t require sedation, and doesn’t require you to stop blood-thinning medications. For people who can’t undergo sedation or are on anticoagulants, it’s a practical alternative. The downside: if something abnormal shows up, you’ll still need a traditional colonoscopy to biopsy or remove it.
Flexible sigmoidoscopy uses a scope similar to a colonoscope but only examines the rectum and the lower third of the colon (the sigmoid and descending colon). It’s shorter and examines less territory than a full colonoscopy. If polyps or signs of cancer are found during a sigmoidoscopy, a full colonoscopy is recommended to check the rest of the colon.
A Blood Test Is Now Available
In 2024, the FDA approved the first blood test for primary colon cancer screening. Called Shield, the test looks for specific changes to DNA fragments floating freely in the bloodstream that signal the presence of a tumor or precancerous growth in the colon. In a study of nearly 8,000 people, it detected colorectal cancer in more than 83% of participants who were confirmed to have cancer on colonoscopy.
The test is less effective at catching precancerous polyps, identifying only about 13% of advanced polyps found by colonoscopy. Its specificity is 90%, meaning 10% of results may falsely suggest a problem. A blood test is the easiest screening option, but its limitations mean it works best as a way to get screened if you’d otherwise skip screening altogether. A positive result still requires a colonoscopy for confirmation.
Symptoms That Lead to Diagnosis
When colon cancer is found because of symptoms rather than screening, it’s often at a later stage. The symptoms that typically trigger diagnostic testing include a persistent change in bowel habits (more frequent diarrhea or constipation that doesn’t resolve), rectal bleeding or blood in the stool, ongoing abdominal discomfort like cramps or gas, a feeling that the bowel doesn’t empty completely, unexplained weight loss, and unusual weakness or fatigue.
None of these symptoms is unique to colon cancer. Many are caused by far more common conditions. But when they persist, particularly rectal bleeding or unexplained weight loss, doctors will typically order a colonoscopy to examine the colon directly and take biopsies of anything abnormal.
How a Diagnosis Is Confirmed
No matter how colon cancer is initially detected, a tissue biopsy is required to confirm the diagnosis. During a colonoscopy, the doctor removes suspicious tissue or polyps and sends them to a pathology lab. A pathologist examines the cells under a microscope and runs a series of tests to determine whether the tissue is cancerous, what type of cancer it is, and what molecular features it has.
Most colon cancers are adenocarcinomas, meaning they start in the cells that line the inside of the colon. The lab also tests the tumor for specific genetic mutations and a feature called microsatellite instability, which influences how the cancer behaves and which treatments are most likely to work. These results shape the entire treatment plan.
Genetic Testing for Inherited Risk
Some colon cancers are driven by inherited genetic conditions, the most common being Lynch syndrome. The CDC recommends genetic testing if your tumor screening results are abnormal, if you’ve had colorectal cancer yourself, if several family members have had cancers associated with Lynch syndrome, or if a family member has already been diagnosed with the condition.
Genetic testing typically starts with a conversation with a genetic counselor who evaluates your personal and family cancer history. If testing reveals Lynch syndrome or another inherited condition, your screening schedule will be significantly more aggressive, and your close relatives may be advised to get tested as well. Identifying these conditions early can be lifesaving for entire families.