A colon cancer screening is a test that looks for cancer or precancerous growths in your colon and rectum before you have any symptoms. The goal is to catch problems early or, better yet, prevent cancer entirely by finding and removing abnormal growths called polyps before they ever become cancerous. Screening is recommended for all average-risk adults starting at age 45.
Why Screening Matters
Colorectal cancer almost always starts as small, abnormal growths (polyps) on the inner lining of the colon or rectum. Most polyps are harmless and never become cancer, but certain types can slowly transform over a period of years. A screening test can find these polyps while they’re still benign, and in the case of a colonoscopy, they can be removed on the spot. That’s what makes colon cancer screening unusual compared to other cancer screenings: it doesn’t just detect cancer early, it can actually prevent it.
The impact is significant. A National Cancer Institute analysis found that of roughly 940,000 colorectal cancer deaths averted over recent decades, 79% were attributed to screening and polyp removal. Treatment advances accounted for just 21%. In other words, screening has done far more to save lives than better treatments have.
Types of Screening Tests
Screening options fall into two main categories: stool-based tests you do at home and visual exams that let a doctor look directly at (or create images of) the inside of your colon.
Stool-Based Tests
These tests check a stool sample for signs of cancer or precancerous polyps. They’re noninvasive, require no preparation, and can be done at home with a kit your doctor provides or mails to you.
- Fecal immunochemical test (FIT): Detects tiny amounts of blood in stool that you wouldn’t see with the naked eye. It’s done once a year. FIT is the most widely used stool test worldwide because it’s simple, inexpensive, and doesn’t require dietary restrictions beforehand.
- Stool DNA test (sold as Cologuard): Looks for both hidden blood and altered DNA shed by abnormal cells in the colon. It’s done every three years. Because it checks for more markers, it tends to catch more precancerous growths than FIT alone, though it also has a higher rate of false positives.
- Guaiac-based fecal occult blood test (gFOBT): An older test that also checks for hidden blood but is less sensitive than FIT. In head-to-head comparisons, FIT detected advanced growths roughly three times more often than the standard gFOBT. This test has largely been replaced by FIT in clinical practice.
If any stool-based test comes back positive, you’ll need a follow-up colonoscopy to investigate further. A positive result doesn’t mean you have cancer. It means something needs a closer look.
Visual Exams
- Colonoscopy: The most thorough option. A doctor uses a long, flexible tube with a camera to examine the entire colon and rectum. Any polyps found during the procedure are typically removed right then. If nothing concerning is found, you won’t need another for 10 years. This is both a standalone screening tool and the follow-up test used after an abnormal result from any other screening method.
- Flexible sigmoidoscopy: Similar to a colonoscopy but examines only the lower third of the colon. It’s less invasive and requires less preparation, but it can miss growths in the upper portions. Research has shown that CT-based imaging detects about twice as many precancerous growths in the upper colon compared to sigmoidoscopy.
- CT colonography (virtual colonoscopy): Uses CT scanning to create detailed images of the colon without inserting a scope. It’s good at spotting larger polyps throughout the entire colon. However, if anything suspicious is found, you’ll still need a standard colonoscopy to remove it.
When to Start and How Often
The U.S. Preventive Services Task Force recommends that all average-risk adults begin screening at age 45. In 2021, the starting age was lowered from 50 to 45, reflecting a troubling rise in colorectal cancer among younger adults. Routine screening is strongly recommended through age 75. Between ages 76 and 85, the decision becomes more individual, based on your overall health and screening history.
How often you screen depends on the test you choose. A colonoscopy with normal results buys you 10 years before your next one. FIT needs to be repeated every year. The stool DNA test is done every three years. All of these schedules assume average risk and normal results. If polyps are found, or if you have certain risk factors, your timeline will be shorter.
What Raises Your Risk
Some people need to start screening before 45 or be screened more frequently. You may fall into a higher-risk category if you have a first-degree relative (parent, sibling, or child) who had colorectal cancer or advanced polyps, particularly if they were diagnosed before age 60. Inflammatory bowel disease, including Crohn’s disease and ulcerative colitis, also raises your risk because long-term inflammation in the colon can lead to cellular changes over time.
Certain inherited genetic conditions carry especially high risk. Lynch syndrome, the most common hereditary cause of colorectal cancer, can mean screening starts as early as your 20s. If you have a strong family history of colorectal or related cancers, genetic counseling can help determine whether earlier or more frequent screening is appropriate.
What Happens During a Colonoscopy
Since colonoscopy is the most common visual screening and the follow-up for all other tests, it helps to know what to expect. The preparation is often considered the hardest part. The day before the procedure, you’ll switch to a clear liquid diet: water, broth, clear juices like apple juice, gelatin, tea or coffee without milk, and sports drinks. Your doctor may ask you to avoid anything with red or purple dye, which can be mistaken for blood during the exam. You’ll also drink a prescribed laxative solution to completely empty your colon.
The procedure itself typically takes 30 to 60 minutes. You’ll receive sedation, so you won’t feel pain and likely won’t remember much. The doctor threads a thin, flexible scope through the rectum and examines the lining of the entire colon. If polyps are found, they’re removed during the same procedure using small instruments passed through the scope. You’ll need someone to drive you home afterward because of the sedation, and most people return to normal activities the next day.
What Polyp Results Mean for You
Not all polyps are the same. After removal, polyps are examined under a microscope to determine their type, and this matters for your follow-up schedule. Polyps fall into two broad categories: those that can become cancerous (neoplastic) and those that cannot (non-neoplastic).
Neoplastic polyps include all adenomas and certain serrated types. These are the ones that could eventually turn into cancer if left in place. Non-neoplastic polyps, like common hyperplastic polyps, carry no cancer risk. If your colonoscopy finds no neoplastic polyps, your next screening is in 10 years. If neoplastic polyps are removed, your doctor will recommend a follow-up colonoscopy in one, three, five, or seven years depending on the number, size, and type of polyps found.
Certain polyp types warrant closer surveillance. Villous adenomas and sessile serrated lesions carry a somewhat higher risk of progressing to cancer, so finding these usually means shorter intervals between screenings.
Cost and Insurance Coverage
Under the Affordable Care Act, all Marketplace health plans and most private insurance plans must cover colorectal cancer screening for adults aged 45 to 75 at no cost to you, meaning no copay, coinsurance, or deductible. This applies when you use an in-network provider. Medicare also covers screening colonoscopies. If a stool test comes back positive and you need a follow-up colonoscopy, recent federal rules have moved toward ensuring that follow-up is also covered without cost-sharing, though coverage details can vary by plan. It’s worth confirming with your insurer before scheduling.