A small sample collected at home may help prevent a deadly outcome years later.
Swedish researchers have found that people who participated in colorectal cancer screening had a much lower risk of dying from the disease than people who did not take part. Their latest estimate suggests the difference could be as large as 43%.
Colorectal cancer, also called bowel cancer, affects the colon or rectum. It is among the most frequently diagnosed cancers and is a major cause of cancer death. Yet it is also a cancer for which early detection can dramatically change the outlook.
When colorectal cancer is discovered before it has spread, treatment is often more successful. The problem is that early disease may cause no warning signs. A person can feel well while small cancers or precancerous changes are already growing inside the bowel.
This makes colorectal cancer especially suitable for screening. Rather than waiting until someone develops bleeding, unexplained weight loss, anemia or persistent bowel changes, screening looks for early signs in people who do not have symptoms. The aim is to catch disease while it is easier to treat.
In Sweden, people aged 60 to 74 are offered screening every two years. The process begins at home rather than in a hospital. A kit is mailed to the person, who collects a small stool sample and sends it for laboratory testing.
The laboratory looks for traces of blood that are too small to see with the naked eye. Cancer and some polyps can bleed slightly, so hidden blood can be a warning sign. A positive test does not diagnose cancer, but it tells doctors that the bowel should be examined more closely.
The usual next step is a colonoscopy. A camera on a flexible tube allows a doctor to inspect the lining of the large bowel. If a polyp is found, it can often be removed before it has the opportunity to develop into cancer.
Researchers at Karolinska Institutet and Umeå University studied one of Sweden’s longest-running screening programs. Stockholm and Gotland began routine colorectal cancer screening in 2008, years before screening became universal across the country. This early start made long-term follow-up possible.
The researchers studied 376,511 people and followed some of them for up to 14 years. During that time, 1,668 deaths from colorectal cancer were recorded. The large number of participants gave the team enough information to examine differences between people exposed to screening and those who were not.
The analysis compared people invited between 2008 and 2012 with a control group that had either not been invited or received an invitation later. This was not as simple as comparing two fixed groups because screening opportunities changed over time. Some people invited for screening did not participate, while some controls later gained access to screening.
The team therefore used statistical methods designed to account for these complications. After adjustment, being offered screening was associated with a 26% lower risk of colorectal cancer death. The estimated benefit was substantially larger for people who actually participated.
Those who completed screening had an estimated 43% lower risk of dying from colorectal cancer. The findings were published in the journal JAMA Network Open. Corresponding author Johannes Blom said the results highlight the importance of taking part rather than simply receiving an invitation.
An earlier evaluation of the program had estimated that people who were invited had a 14% lower mortality risk. The new analysis had longer follow-up and used methods intended to provide a clearer picture of the effect of screening. This helped researchers estimate both the effect of invitation and the effect associated with actual participation.
The participation issue is particularly important because about one in three people offered screening do not send back a stool sample, even though the Swedish program is free. That means a large number of eligible adults may be missing an opportunity to detect cancer or precancerous changes early.
Some people may avoid screening because collecting stool feels unpleasant or embarrassing. Others may assume that screening is unnecessary when they have no bowel symptoms. But the entire purpose of screening is to find hidden disease before symptoms appear.
Another reason people may hesitate is fear of what happens after a positive test. Most positive stool tests do not automatically mean cancer is present, and several noncancerous conditions can cause bleeding. Colonoscopy is used to find the source and can sometimes prevent future cancer by removing risky polyps.
The study provides strong real-world evidence because it involved hundreds of thousands of people and more than a decade of follow-up. Sweden’s national health registers also allowed researchers to track deaths reliably. These are major advantages when studying whether a screening program has a lasting effect.
However, the study is not the same as a perfectly controlled randomized trial of participation. People who choose to complete screening may be more health-conscious than people who decline it. They may be more likely to exercise, attend medical appointments or follow other preventive advice, and those differences could influence mortality.
The researchers used statistical adjustments to reduce these sources of bias, but some uncertainty remains. For this reason, the exact 43% figure should not be interpreted as a guarantee that screening reduces every individual’s risk by precisely that amount. It is an estimate based on population data.
Even with that caution, the findings reinforce the broader evidence supporting colorectal cancer screening. Detecting cancer earlier can improve treatment options, while finding and removing certain polyps can stop some cancers before they start. Few cancer prevention strategies offer both early detection and the possibility of removing a precursor to disease.
The study also points to a public health opportunity. If health systems can understand why people ignore or decline screening kits, they may be able to increase participation through clearer information, reminders and easier testing. Reaching people who currently do not participate could potentially save additional lives.
The overall message is not that a stool test prevents every colorectal cancer death. Screening can miss some cancers, and no medical test is perfect. But this long-running Swedish program suggests that taking part when screening is offered may provide a substantial survival benefit, making a few minutes spent completing a home test potentially very important years later.
Source: Karolinska Institutet and Umeå University.
