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When given the option, patients ages 45-49 completed colorectal cancer (CRC) screening via fecal immunochemical test (FIT) at about the same rate as 50-year-olds, according to a large retrospective cohort study.

Among more than 267,000 patients who received a FIT kit in 2022, the proportion who completed it within 3 months was 39% in those age 45-49 versus 38% in 50-year olds (RR 1.05, 95% CI 1.04-1.06), a research team led by Theodore Levin, MD, of Kaiser Permanente Northern California in Pleasanton, California, reported online in the .

The FIT results came back positive for 3.6% of the younger group and for 4.0% of those age 50 (RR 0.91, 95% CI 0.84-0.98). The percentage of patients who underwent colonoscopy after a positive test did not differ significantly between the two groups (65% vs 67%; RR 1.00, 95% CI 0.94-1.05).

While adenoma was detected at lower rates in the younger group (59% vs 67%; RR 0.88, 95% CI 0.83-0.95), there were similar rates of detection of CRC (2.8% vs 2.7%), adenoma with advanced histology, polyp with high-grade dysplasia, and sessile serrated lesions.

"Overall, our finding that the prevalence of colorectal neoplasia in patients aged 45 to 49 years approaches rates in those aged 50 years suggests that CRC risk levels are likely similar in both groups," Levin and colleagues wrote.

"Also, the generally similar yields in the two age groups, as well as across age years versus age 50 years, provide strong support for lowering the start of screening initiation to age 45 years and for using a noninvasive test to select patients for colonoscopy in this younger age group," they concluded.

Over the past several years, the American Cancer Society, the U.S. Preventive Services Task Force, and the American College of Gastroenterology shifted the age to start CRC screening from 50 to 45. The younger age threshold was based on evidence of increasing disease burden in younger adults, emerging data showing the prevalence of advanced colorectal neoplasia in individuals age 45-49 approaches rates in those age 50-59, and modeling studies showing screening benefits outweigh potential harms, Levin and colleagues explained.

However, the guidelines acknowledged a lack of available data on screening completion and outcomes in patients ages 45-49, Levin's group noted. Notably, the American College of Physicians has stuck with its recommendation to start screening at age 50 in asymptomatic, average-risk adults.

The new study evidence is "convincing," said Jeffrey Fox, MD, a spokesperson for the American Gastroenterological Association who was not involved in the study. "The geographical breadth of the target population and size of the sample should be sufficient to detect differences between the 45-49 and 50 year old groups. The fact that there was no drop in the adherence rates or neoplasm finding capabilities of FIT in the younger group is encouraging and supports its effectiveness for screening younger adults."

However, he added: "It is less clear whether conventional screening methods would have similar performance characteristics in this newly screened younger population, where the incidence of cancer is lower and awareness for the importance of cancer screening might be lower as well."

The retrospective study included a cohort of 267,732 patients in Kaiser Permanente Northern California, Washington, and Colorado health systems. Patients were excluded if they had inflammatory bowel disease, prior lower gastrointestinal surgery, or a history of CRC. The majority (80%) were age 45-49, and 20% were age 50.

All patients received a FIT kit by mail or during an office visit from January to September 2022. Patients were excluded if they had previously received a FIT kit or underwent colonoscopy, so the FIT kits distributed in 2022 were the first documented CRC screening opportunity for each patient.

The primary study outcomes included FIT completion, defined as completing the test within 3 months of the distribution date; FIT positivity, defined as the proportion of tests with a positive result; follow-up colonoscopy completion, defined as having a colonoscopy within 3 months of a positive FIT result; and findings of the follow-up colonoscopy.

The relatively short 3-month follow-up was a limitation of the study, the authors said. A longer follow-up would likely have resulted in higher completion rates and possibly more differences between the groups. Restricting the study to patients ages 45-50 was another limitation, they added, because including older or younger patients might have allowed them to find the inflection point for CRC risk.

"Finally, an assessment of the policy significance of these findings should take into account that the study was not specifically designed to directly compare the strategies of initiating CRC screening at age 45 versus 50 years," the researchers said.

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