Finding out whether you have colon cancer starts with screening, even if you have no symptoms. Most colon cancers develop slowly from small growths called polyps, and the right test at the right time can catch them years before they cause problems. The process typically begins with either a simple stool test you do at home or a colonoscopy, depending on your age, risk factors, and preferences.
Symptoms That Should Prompt Testing
Colon cancer often causes no symptoms in its early stages, which is why routine screening matters so much. When symptoms do appear, the most common ones include a persistent change in bowel habits (more frequent diarrhea or constipation that doesn’t resolve), rectal bleeding or blood in the stool, and ongoing belly discomfort like cramps, gas, or pain. You might also notice unexplained weight loss, fatigue, or a feeling that your bowel doesn’t fully empty.
Blood in the stool can look bright red or make stool appear very dark. Either warrants attention. None of these symptoms automatically means cancer, as many are caused by hemorrhoids, infections, or irritable bowel syndrome. But if any persist for more than a couple of weeks, they’re worth investigating rather than waiting for your next routine screening.
When Screening Should Start
The U.S. Preventive Services Task Force recommends that adults begin screening for colorectal cancer at age 45 and continue through age 75. Between 76 and 85, the decision becomes more individual based on your overall health and screening history.
Some people need to start much earlier. If you have a family history of colon cancer or carry certain genetic conditions, the timeline shifts significantly. People with Lynch syndrome, which causes polyps that can turn cancerous quickly, are advised to begin colonoscopies between ages 20 and 25. Those with familial adenomatous polyposis (FAP), a condition that can produce hundreds or thousands of polyps, typically start colonoscopies by age 10 to 12. If a close relative was diagnosed young, your screening may begin five years before their age at diagnosis.
At-Home Stool Tests
If you’re at average risk and prefer to start with something less invasive, stool-based tests can detect signs of cancer or precancerous growths from a sample you collect at home. Two main options exist, and they differ considerably in accuracy.
The fecal immunochemical test (FIT) looks for hidden blood in your stool. It detects stage I through III colon cancers about 64.6% of the time. It’s inexpensive, widely available, and has a relatively low false-positive rate of around 4.3%, meaning few people get an unnecessary scare.
The multi-target stool DNA test (sold as Cologuard) checks for both blood and DNA changes shed by abnormal cells. It catches stage I through III cancers at a rate of 92.7%, a substantial improvement over FIT. It also picks up advanced precancerous growths about 43% of the time compared to FIT’s 23%. The tradeoff is a higher false-positive rate: roughly 7% of people who test positive turn out to have nothing concerning on follow-up colonoscopy.
A positive result on either test does not mean you have cancer. It means something in your stool triggered the test, and you need a colonoscopy to find the actual cause. Nearly all people who test positive are advised to have that follow-up colonoscopy within a year.
Colonoscopy: The Definitive Test
Colonoscopy remains the most thorough way to examine the colon. A long, flexible tube with a tiny camera is guided through the rectum and the entire length of the colon, giving the doctor a direct view of the lining. If any polyps or suspicious tissue are found, they can be removed on the spot and sent to a lab for analysis. This means colonoscopy is both a screening tool and a treatment in one visit.
Preparation is the part most people dread, and it starts days before the procedure. You’ll eat a low-fiber diet for two to three days, switch to clear liquids on the final day, then take a laxative prep the evening before. Many prep regimens now use a split dose, where you take half the evening before and the second half early the morning of the procedure. Newer tablet-based preps and lower-volume liquid options have made this more manageable than it used to be, with some requiring as little as 10 ounces of prep solution.
The procedure itself typically takes 30 to 60 minutes, and you’ll be sedated, so you won’t feel it. You will need someone to drive you home afterward. Most people return to normal activities the next day.
Virtual Colonoscopy as an Alternative
If you can’t or don’t want to undergo a traditional colonoscopy, a virtual colonoscopy (CT colonography) uses a CT scanner to create detailed images of the colon without inserting a scope. It finds large polyps and cancers at roughly the same rate as a traditional colonoscopy. It’s a reasonable option if you want to avoid sedation, need to drive yourself home, have a bleeding disorder, or have a bowel blockage that makes traditional colonoscopy risky.
The catch: if the scan finds something suspicious, you’ll still need a traditional colonoscopy to remove it. And you still have to do the bowel prep beforehand.
What Happens When Polyps Are Found
Not all polyps are dangerous, and knowing the type matters. After removal, polyps go to a pathology lab, and the results determine your next steps.
- Adenomatous polyps (adenomas) are the most common precancerous type. These are the polyps most likely to become colon cancer over time, and removing them is the primary reason colonoscopy prevents cancer.
- Sessile serrated lesions are flat polyps, often found in the right side of the colon. They can be hard to spot but may turn cancerous, especially when large or showing irregular cell growth.
- Traditional serrated adenomas are rare but clearly precancerous and always removed.
- Hyperplastic polyps are generally harmless and don’t become cancerous on their own.
- Inflammatory polyps don’t turn into cancer directly, but they signal chronic inflammation in the colon, which raises overall risk.
If precancerous polyps are found, your doctor will recommend a follow-up colonoscopy sooner than the standard 10-year interval, often in three to five years depending on the number, size, and type of polyps removed.
If Cancer Is Confirmed
When a biopsy confirms cancer cells, the next step is staging, which determines how far the cancer has spread. This involves imaging tests to see whether the tumor has grown through the colon wall, reached nearby lymph nodes, or spread to other organs like the liver or lungs. The stage, ranging from I (confined to the inner lining) to IV (spread to distant organs), shapes the entire treatment plan and provides the clearest picture of prognosis.
Early-stage colon cancers caught through screening have significantly better outcomes. That gap between stage I and stage IV is precisely why screening exists: catching growths before they become cancer, or catching cancer before it spreads, changes the trajectory entirely.