Stool Tests and Colonoscopies Catch Colorectal Cancer Earlier in 278,000-Person Trial

The Central Question Sweden Spent a Decade Answering

Colorectal cancer kills roughly 900,000 people annually worldwide. It is also, in many cases, entirely preventable. The colon is accessible; its precancerous growths, called adenomas, can be seen and removed; and its cancers, when caught early, are highly treatable. These facts make a compelling case for screening -- but the specifics matter enormously. Which test? How often? At what age? With what tradeoffs?

Sweden's SCREESCO study set out to answer those questions with a design rigorous enough to settle the debate. Starting in 2014, the trial enrolled more than 278,000 adults who were exactly 60 years old at enrollment, drawn directly from the population register. They were randomized into three groups: one offered a single colonoscopy, one offered two rounds of fecal immunochemical testing (FIT) in years one and three, and one receiving no intervention as a control. Results through 2020, published in Nature Medicine, now provide the first large-scale comparative data on how well these approaches shift cancer diagnoses toward earlier stages.

What Both Tests Accomplished

Both screening approaches outperformed no screening at detecting cancer at an early, localized stage. The effect was most visible in the first two years -- the period when most of the interventions were actually completed. "The results show that both types of screening lead to more cases of early-stage cancer being detected, especially in the first two years," said Marcus Westerberg, docent at Uppsala University and the study's corresponding author.

Early detection matters because it translates directly into survival. Colorectal cancer caught at stage I carries a five-year survival rate above 90%; caught at stage IV, survival rates drop below 15%. Shifting diagnoses toward earlier stages is not a procedural footnote -- it is the mechanism by which screening programs save lives.

The FIT Advantage

Among the two testing methods, the FIT arm showed the most striking signal. By the end of follow-up, 0.61% of people in the FIT group had developed colorectal cancer, compared to 0.73% in the unscreened control group. That difference -- 12 people per 10,000 -- suggests the test may be doing more than finding cancer early. It may be preventing some cancers from forming at all.

The mechanism is the colonoscopy that follows a positive FIT result. When a stool sample shows blood above the threshold of 10 micrograms of hemoglobin per gram, patients undergo a colonoscopy. If adenomas are found, they are removed. Those growths never become cancer.

"We can show that cases of advanced cancer tend to decrease towards the end of the period in the group randomised to provide stool samples for FIT. This could be evidence of a preventive effect of the screening, through the removal of the precursors to cancer," said Anna Forsberg, docent at Karolinska Institutet and SCREESCO's lead investigator.

The Risks of More Colonoscopies

Colonoscopy is not without risk. Both screening arms required more colonoscopies than the unscreened group, and the study documented a slight increase in gastrointestinal bleeding and blood clots in those groups, concentrated in the first year when most procedures were performed. These complications were uncommon, and all-cause mortality was identical across all three groups through follow-up. No lives were lost to the screening process in aggregate terms.

That finding is reassuring, but the numbers require careful interpretation. The follow-up period extends only to 2020, roughly six years after enrollment. Colorectal cancer screening trials typically require at least a decade to show mortality benefit. SCREESCO has not yet shown that screening saves lives outright -- only that it shifts detection earlier and reduces advanced-disease rates.

The Path to 2030

The SCREESCO consortium plans to track all participants until 2030. "This study gives us great hope that screening will also be shown to reduce mortality from colorectal cancer in both groups when the study is finally evaluated in about five years," said Forsberg.

Sweden has already been running a national FIT-based screening program since 2021, with full rollout planned for 2026, targeting adults aged 60 to 74. The SCREESCO data -- from the same population, a lower FIT threshold, and a randomized design -- will help calibrate and refine that program as more follow-up data accumulate.