When to start colonoscopy screening? A massive study backs starting at 45

If you received a colonoscopy reminder in the mail when you turned 50, you’re not alone. For decades, age 50 was the magic number to begin colorectal cancer screening in the U.S. But with rising numbers of cases among younger adults, that long-standing benchmark is now being challenged. A new study led by Drs. Carolyn Rutter and Nascimento de Lima from Fred Hutchinson Cancer Center and RAND Corporation puts current screening guidelines through a scientific “stress test”—and the results make a compelling case for starting earlier.

Colorectal cancer (CRC) remains one of the leading causes of cancer-related deaths in the U.S., even as overall death rates have declined thanks to improvements in screening, treatment, and healthier behaviors. But beneath that progress lies a troubling trend: more people under 50 are being diagnosed. Between the mid-1990s and 2019, the proportion of CRC cases in adults under 55 nearly doubled. People born around 1990 now face twice the risk of colon cancer—and four times the risk of rectal cancer—compared to those born in 1950. Researchers aren’t certain why this is happening, but lifestyle factors like diet, alcohol use, obesity, and metabolic changes are likely contributors. Whatever the cause, the upward trend has led many experts to reconsider when screening should begin.

At the heart of the debate are two major guidelines. The U.S. Preventive Services Task Force (USPSTF) now recommends starting screening at age 45. Meanwhile, the American College of Physicians (ACP) still holds to the traditional advice of beginning at 50. To evaluate which strategy holds up better under scrutiny, the researchers turned to a sophisticated simulation tool called CRC-SPIN—the Colorectal Cancer Simulated Population model for Incidence and Natural History. Think of it as a virtual lab that lets scientists model millions of lifespans and see how different screening schedules impact outcomes.

These tools make assumptions about how CRC develops over time. For example, how quickly precancerous polyps progress to cancer, how sensitive screenings are at detecting early disease, and how people behave in terms of showing up for tests. Because no single model can capture all real-world complexity, researchers used a wide range of biological profiles, disease progression rates, and test performance assumptions to test how recommendations would hold up under different scenarios.

What set this study apart wasn’t just its conclusion; it was the scale and depth of its analysis. Rather than relying on a single model or a fixed set of assumptions, the researchers created a virtual testing ground with remarkable complexity. They ran 105,000 different simulations, combining 500 different biological profiles, two models of how cancer might develop, and four versions of how accurate colonoscopies might be in real-world settings. Each simulation modeled five million individuals, amounting to over half a trillion synthetic life histories. This method, called robust decision-making, is all about asking: what if the disease behaves differently than we think? What if screening isn’t as accurate as we hope? And through all that uncertainty, does the USPSTF recommendation still hold?

It did. No matter how the variables shifted, starting screening at age 45 consistently led to better outcomes. The ACP’s more conservative approach—starting at 50 and screening every 10 years—never came out ahead. In fact, it often required just as many colonoscopies while saving fewer lives. Even when colonoscopy performance was downgraded to reflect imperfect clinical practice, the 45-start strategy still delivered meaningful benefits. This isn’t just a statistical win; it’s a signal to health systems, insurers, and guideline committees that earlier screening may offer better value across the board.

Perhaps most importantly, the study didn’t just focus on raw outcomes; it asked how efficiently those outcomes were achieved. Starting at 45 not only saved more lives, but it did so with fewer procedures per life saved. Screening more frequently or continuing well into someone’s 80s offered little additional benefit but significantly ramped up procedure volume. For public programs and patients alike, these diminishing returns matter. When resources are limited and when procedures aren’t exactly fun, strategies that do more with less should rise to the top.

These findings matter not just in public health offices and policy circles, but in the exam room. If you’re in your early-to-mid 40s, this might feel like just one more thing to worry about. A colonoscopy isn’t high on anyone’s wish list. But this research makes a clear, evidence-backed argument: starting screening at 45 isn’t just reasonable, it’s smart. It saves lives, uses resources more wisely, and holds up under just about any scenario we can imagine. So, if you’re approaching that age, now’s the time to bring it up with your doctor. It’s a simple conversation that could make a big difference for you or someone you care about.

Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium Member Dr. Carolyn Rutter contributed to this research.

This research was supported by the National Cancer Institute as part of the Cancer Intervention and Surveillance Modeling Network (CISNET), and a Rothenberg Dissertation Award provided by the Pardee RAND Graduate School.

Nascimento de Lima, P., Maerzluft, C., Ozik, J., Collier, N., & Rutter, C. M. (2025). Stress-testing US colorectal cancer screening guidelines: Decennial colonoscopy from age 45 is robust to natural history uncertainty and colonoscopy sensitivity assumptions. Medical decision making : an international journal of the Society for Medical Decision Making, 272989X251334373. Advance online publication.