Colorectal Cancer Screening: When to Start and Which Test

Key points#

• Two US bodies now agree that screening should begin at age 45 for average-risk adults. The USPSTF gives a B recommendation for ages 45 to 49 and an A recommendation for ages 50 to 75, and the American Cancer Society moved its qualified starting point to 45 in 2018.
• There is no single best test. Stool-based tests (FIT annually, multitarget stool DNA every 3 years) and visual or structural tests (colonoscopy every 10 years, CT colonography and flexible sigmoidoscopy every 5 years) are all endorsed. The best test is the one a person will actually complete on schedule.
• Any positive result on a non-colonoscopy test must be followed by a timely colonoscopy. Without that step the screening is incomplete, and this is the point patients most often miss.
• The upper age limit is individualized. The USPSTF recommends selectively offering screening for ages 76 to 85 based on health, life expectancy, and prior screening, rather than a fixed stop age for everyone.
• People at higher-than-average risk (a personal or family history of colorectal cancer or advanced polyps, inflammatory bowel disease, or a hereditary syndrome such as Lynch syndrome) fall outside this pathway and need an individualized plan.

If you are at average risk, colorectal cancer screening should begin at age 45, and there is no single best test. The main choice is between a colonoscopy every 10 years and a home stool test (annual FIT, or a multitarget stool DNA test every 3 years), and the guidelines endorse either path.

The practical answer is that the best test is the one you will actually complete on schedule. A colonoscopy is the most sensitive single test and can remove precancerous polyps in the same session, while stool tests are noninvasive and done at home but need repeating more often. Either is a correct choice, as long as any positive stool test is followed by a colonoscopy. The rest of this guide covers who counts as average risk, how the options compare, and how to pick the one you will finish.

Why colorectal cancer screening matters, and why the starting age changed#

Colorectal cancer is among the most common causes of cancer death among US adults. What sets it apart from most cancers is that screening can prevent it, not merely catch it early. Most colorectal cancers grow from adenomatous polyps over a period of years, and a screening test that finds and removes a polyp interrupts that sequence before a cancer ever forms.

The starting age moved recently, and it is worth understanding why. For most of the last two decades, average-risk screening began at 50. Then incidence in adults under 50 started climbing, a trend that has held across several years of data. In response, the American Cancer Society lowered its recommended starting age to 45 in its 2018 guideline update, framing 45 as a qualified recommendation. The US Preventive Services Task Force reached the same conclusion in its 2021 statement.

This sits in a men's-health series because prevention visits are a common reason men come in, and screening is often the concrete thing that comes out of them. But nothing about the guidance is specific to men. It applies to all average-risk adults, and the numbers below hold for women as well.

When to start, and who counts as average risk#

"Average risk" has a precise meaning in the USPSTF statement, and it is worth reading carefully because the whole timeline below assumes it. You are average risk if you have no prior colorectal cancer, no history of adenomatous polyps, and no inflammatory bowel disease, and if you have no personal or family history of a hereditary syndrome that raises lifetime risk, such as Lynch syndrome or familial adenomatous polyposis.

For people who meet that definition, the age brackets and their strength of recommendation are as follows.

• Ages 45 to 49: begin screening. This is the USPSTF B recommendation, meaning the evidence supports a moderate net benefit.
• Ages 50 to 75: the strongest benefit. This is the A recommendation, where the evidence is most consistent.
• Ages 76 to 85: individualized. Screening is offered selectively rather than routinely, based on the factors described later in this post.

If you do not fit that definition, this general timeline is not yours to follow. A personal or family history of colorectal cancer or advanced polyps, inflammatory bowel disease, or a known hereditary syndrome all change the plan, often meaning an earlier start and shorter intervals, and call for a specific conversation with your clinician rather than a default schedule.

The stool-based options: FIT, guaiac FOBT, and multitarget stool DNA#

Both guidelines endorse at-home stool tests. They differ in what they detect and how often you repeat them.

• Fecal immunochemical test (FIT), annually. FIT detects human hemoglobin in stool, a marker of bleeding that can come from a polyp or cancer. One sample, collected at home, repeated every year. The older high-sensitivity guaiac-based fecal occult blood test also looks for blood in the stool and is likewise used yearly.
• Multitarget stool DNA test, every 3 years. This combines a FIT-type test for blood with markers of altered DNA shed from the colon lining, and it is repeated every 3 years.

The appeal of all three is the same: no bowel preparation, no sedation, no clinic visit for the test itself. The trade-off is also shared. Stool tests are repeated more often and are less sensitive for precancerous lesions than colonoscopy, so they flag cancer better than they catch a polyp that has not yet started to bleed or shed abnormal cells.

The multitarget stool DNA test has two pivotal trials worth citing plainly. The 2014 trial reported high sensitivity for cancer but lower specificity than FIT, meaning more false positives. The 2024 next-generation trial (BLUE-C) reported a similar pattern with the updated test: strong cancer sensitivity, and specificity still below FIT. A false positive here is not harmless, because it leads to a colonoscopy that, in retrospect, was not needed. Both trials were funded by the test's manufacturer, which is a limitation to weigh when reading the numbers rather than a comment on the investigators. Industry funding is common in diagnostics research and simply means the results deserve the same scrutiny you would apply to any single-sponsor study.

The visual and structural options: colonoscopy, CT colonography, flexible sigmoidoscopy#

Three tests look at the colon directly or by imaging.

• Colonoscopy, every 10 years. A flexible scope examines the entire colon.
• CT colonography, every 5 years. A CT scan reconstructs the colon lining after bowel preparation and air insufflation.
• Flexible sigmoidoscopy, every 5 years (or every 10 years combined with annual FIT in some protocols). A shorter scope examines the lower colon.

Colonoscopy has one distinguishing advantage: it can find and remove a polyp in the same session, so a suspicious finding is often dealt with on the spot rather than scheduled for a return visit. A normal exam also buys a long interval, ten years for average-risk adults. Against that, the burdens are real: a full bowel preparation the day before, usually sedation on the day, time away from work, and a small risk of complications such as bleeding or perforation.

CT colonography and flexible sigmoidoscopy are valid alternatives on the ACS and USPSTF option lists. The one rule that applies to both, and to every stool test above, is simple: a positive or abnormal result requires a follow-up colonoscopy. These tests screen; colonoscopy is what evaluates and treats.

How to choose: matching the test to the person#

Here is the part that actually decides outcomes. A theoretically superior test that a person keeps postponing protects no one, so the decision is really a fit between a test and a person rather than a ranking of tests. Someone who wants a one-and-done option, does not mind the preparation, and would rather not think about it again for a decade is often well served by colonoscopy. Someone who wants to avoid a procedure, or who cannot easily take a day off, may be far better off starting with an annual FIT that they will actually complete than with a colonoscopy they will defer indefinitely. Both are correct answers.

Two things belong in every version of this conversation. First, a stool test is only complete if a positive result is followed by a colonoscopy. Choosing FIT means committing in advance to that follow-up step, because a positive FIT that is never worked up is worse than not having tested. Second, insurance coverage, local availability, and personal preference are legitimate inputs, not distractions from the "real" clinical question. The clinical question includes them.

This is ordinary generalist reasoning: hold the evidence, the person's values, and the odds of follow-through in view at once, and land on the plan most likely to end in a completed screen. It is the same weighing that family medicine and internal medicine bring to most preventive decisions.

After a normal result, and knowing when to stop#

Rescreening depends on the test and the result. A normal colonoscopy generally means a 10-year interval. A negative FIT means repeat in a year. A negative multitarget stool DNA test means repeat in 3 years. If a stool test is positive and the follow-up colonoscopy is normal, your clinician will set the next interval based on that colonoscopy.

The upper age question deserves an honest answer rather than a single number. The evidence for benefit is strongest through age 75. From 76 to 85, the USPSTF recommends offering screening selectively, weighing overall health, life expectancy, and prior screening history. A healthy 78-year-old who has never been screened is a different case from an 84-year-old with several serious conditions and a lifetime of normal colonoscopies. Starting screening for the first time after 85 is generally not recommended. Framed correctly, stopping is a conversation, not an automatic cutoff at a birthday.

One point stands apart from all of this. Screening is for people without symptoms. New rectal bleeding, a persistent change in bowel habits, or unexplained anemia warrants medical evaluation regardless of when you were last screened and regardless of your age. A recent normal colonoscopy does not mean a new symptom can wait.

Booking the one you will finish#

Pick the test you can see yourself completing, then complete it. If that is a colonoscopy, schedule it and clear the day. If it is an annual FIT, put a standing reminder on the calendar and commit now to the colonoscopy that a positive result would require. The worst outcome here is not choosing the "wrong" test; it is the well-intentioned plan that never gets finished. Bring your age, your family history, and your honest sense of what you will actually do to your next primary care visit.